user17628: What is hasituppāda, the so-called rootless, mirth-producing, mind-consciousness element of an Arahant? What is its basis? What is its purpose, if any? Could someone provide me the details of the same with references from the Pāli Canon?

user471: From the dictionary entry on "hasituppāda-citta":

hasituppāda-citta：This term is used in Abh．S．for the citta
，Tab．I，72．This type of consciousness （the Buddha's smile） is often
implied in the suttas.
hasituppāda-citta：lit．'consciousness producing mirth' （smile），is found
in the Abhidhammatthasaṅgaha as a name for the joyful
mind-consciousness element （manoviññāṇa-dhātu，Tab．I．72） arising as
functional consciousness independent of kamma （kiriya-citta），only in
the Arahat．- （App.）.

From the dictionary entry on "hasita":

Hasita Hasita [pp. of hasati, representing both Sk. hasita & hṛṣita]
laughing, merry; (nt.) laughter, mirth A i.261; Pv iii.35 (=hasitavant
hasita-mukhin C.); Miln 297; Bu i.28; J i.62 (? read hesita); iii.223;
Vism 20.
-uppāda "genesis of mirth," aesthetic faculty Tikp 276; see Cpd. 20 sq.

From the dictionary entry on "uppāda":

Uppāda Uppāda2 [Sk. utpāda, ud + pad] coming into existence,
appearance, birth Vin i.185; D i.185; S iii.39 (+ vaya); iv.14; v.30;
A i.152 (+ vaya), 286, 296; ii.248 (taṇh˚); iii.123 (citt˚ state of
consciousness); iv.65 (id.); Dh 182, 194; J i.59, 107 (sat˚); Vbh 303
(citt˚), 375 (taṇh˚); PvA 10; ThA 282. — anuppāda either "not coming
into existence" D iii.270, M i.60; A i.286, 296; ii.214, 249: iii.84
sq.; Ps i.59, 66; Dhs 1367; or "not ripe" D i.12.

So "hasituppāda-citta" refers to the mental state of an arahant that generates a smile. This term had apparently originated in the Abhidhamma.
Please see the questions "Are there examples of the Buddha showing humour in the Pali Canon?" and "The Buddha and the act of smiling". Sutta or commentary examples of "hasituppāda-citta" can be found in the answers to these questions.
The explanation on hasituppāda-citta comes in this answer by Ven. Yuttadhammo:

You could say the Buddha and arahants had a peculiar form of
humour.... The hasituppādacitta (smiling-producing mind) is a citta
unique to enlightened beings. While they can also smile due to
beautiful-functional cittas, the hasituppādacitta is rootless,
containing none of the wholesome or unwholesome roots.
This means they would smile at things ordinary folk wouldn't, like
brahmas being born as pigs (Dhp-A 338), or ghosts
being tortured for past misdeeds (Dhp-A 71).


17628: <p>What is hasituppāda, the so-called rootless, mirth-producing, mind-consciousness element of an Arahant? What is its basis? What is its purpose, if any? Could someone provide me the details of the same with references from the Pāli Canon?</p>
 471: <p>From the dictionary entry on &quot;<em><a href="https://dictionary.sutta.org/browse/h/hasitupp%C4%81da-citta/" rel="nofollow noreferrer">hasituppāda-citta</a>&quot;</em>:</p>
<blockquote>
<p><em>hasituppāda-citta</em>：This term is used in Abh．S．for the <em>citta</em>
，Tab．I，72．This type of consciousness （the Buddha's smile） is often
implied in the suttas.</p>
<p><em>hasituppāda-citta</em>：lit．'consciousness producing mirth' （smile），is found
in the <em>Abhidhammatthasaṅgaha</em> as a name for the joyful
mind-consciousness element （<em>manoviññāṇa-dhātu</em>，Tab．I．72） arising as
functional consciousness independent of <em>kamma</em> （<em>kiriya-citta</em>），only in
the <em>Arahat</em>．- （App.）.</p>
</blockquote>
<p>From the dictionary entry on &quot;<em><a href="https://dsal.uchicago.edu/cgi-bin/app/pali_query.py?qs=hasita&amp;searchhws=yes" rel="nofollow noreferrer">hasita</a></em>&quot;:</p>
<blockquote>
<p><em>Hasita Hasita</em> [pp. of <em>hasati</em>, representing both Sk. <em>hasita</em> &amp; <em>hṛṣita</em>]
laughing, merry; (nt.) laughter, mirth <a href="https://suttacentral.net/an3.107/en/sujato" rel="nofollow noreferrer">A i.261</a>; Pv iii.35 (=<em>hasitavant</em>
<em>hasita-mukhin</em> C.); Miln 297; Bu i.28; J i.62 (? read <em>hesita</em>); iii.223;
Vism 20.<br />
-<em>uppāda</em> &quot;genesis of mirth,&quot; aesthetic faculty Tikp 276; see Cpd. 20 sq.</p>
</blockquote>
<p>From the dictionary entry on &quot;<em><a href="https://dsal.uchicago.edu/cgi-bin/app/pali_query.py?qs=upp%C4%81da&amp;searchhws=yes" rel="nofollow noreferrer">uppāda</a></em>&quot;:</p>
<blockquote>
<p><em>Uppāda Uppāda2</em> [Sk. <em>utpāda</em>, <em>ud</em> + <em>pad</em>] coming into existence,
appearance, birth Vin i.185; D i.185; S iii.39 (+ <em>vaya</em>); iv.14; v.30;
A i.152 (+ <em>vaya</em>), 286, 296; ii.248 (<em>taṇh</em>˚); iii.123 (<em>citt</em>˚ state of
consciousness); iv.65 (id.); Dh 182, 194; J i.59, 107 (<em>sat</em>˚); Vbh 303
(<em>citt</em>˚), 375 (<em>taṇh</em>˚); PvA 10; ThA 282. — <em>anuppāda</em> either &quot;not coming
into existence&quot; D iii.270, M i.60; A i.286, 296; ii.214, 249: iii.84
sq.; Ps i.59, 66; Dhs 1367; or &quot;not ripe&quot; D i.12.</p>
</blockquote>
<p>So &quot;<em>hasituppāda-citta</em>&quot; refers to the mental state of an arahant that generates a smile. This term had apparently originated in the Abhidhamma.</p>
<p>Please see the questions &quot;<a href="https://buddhism.stackexchange.com/q/8093/471">Are there examples of the Buddha showing humour in the Pali Canon?</a>&quot; and &quot;<a href="https://buddhism.stackexchange.com/q/43929/471">The Buddha and the act of smiling</a>&quot;. Sutta or commentary examples of &quot;<em>hasituppāda-citta</em>&quot; can be found in the answers to these questions.</p>
<p>The explanation on <em>hasituppāda-citta</em> comes in <a href="https://buddhism.stackexchange.com/a/8119/471">this answer</a> by <a href="https://en.wikipedia.org/wiki/Yuttadhammo_Bhikkhu" rel="nofollow noreferrer">Ven. Yuttadhammo</a>:</p>
<blockquote>
<p>You could say the Buddha and arahants had a peculiar form of
humour.... The <em>hasituppādacitta</em> (smiling-producing mind) is a <em>citta</em>
unique to enlightened beings. While they can also smile due to
beautiful-functional cittas, the <em>hasituppādacitta</em> is rootless,
containing none of the wholesome or unwholesome roots.</p>
<p>This means they would smile at things ordinary folk wouldn't, like
brahmas being born as pigs (<a href="https://www.tipitaka.net/tipitaka/dhp/verseload.php?verse=338" rel="nofollow noreferrer">Dhp-A 338</a>), or ghosts
being tortured for past misdeeds (<a href="https://www.tipitaka.net/tipitaka/dhp/verseload.php?verse=071" rel="nofollow noreferrer">Dhp-A 71</a>).</p>
</blockquote>
