stone cold nuts
The above definition of the "nuts" is incorrect.  The nuts is the best possible hand.  So, contrary to the above, if a straight flush (or full house, etc.)  is possible, then an ace-high (non royal) flush is *not* the nuts. If someone says they have the nuts but another possible hand could beat them, they are wrong.

Things I am not sure about (below refers to holdem):

1) Often (but not always) the nuts = "nuts, given the face up cards" and does *not* (this is the controversial point) demand that all the cards 5 community are showing; i.e., in holdem, it is often thought possible to have the nuts on the flop (and/or turn), even if another hand could improve to beat you (but no hand can now beat you). 

NB: If one can have the nuts with more cards to come, we can perhaps distinguish between nuts and "unbreakable" nuts. The question is if all nuts are (implicitly) unbreakable. 

2)Stone cold nuts means either A) unbreakable nuts, and/or B) un-tie-able nuts; The nuts that not only cannot be beat, but cannot be tied.  If I have the nut straight, I cannot be beat, but I can be tied. If I have the nut flush, I cannot be tied. 

I have heard stone cold nuts = to nut hand that cannot be tied *and* wins the entire pot.  But, it seems like the stone cold nuts might fail to win the entire pot, if one is all in with them before the side pots are built. So, I don’t like this definition.

Basically, SCNs > Nuts, but I can think of three ways they could be better: a) unbreakable, b) un-tie-able, c) entire-pot-winning. If we conclude c is wrong, and if we go with b, that allows us to have: nuts, unbreakable nuts, stone cold nuts, and STONE COLD UNBREAKABLE NUTS.  All delicious to squirrels like me. 

Finally, there are the two possible histories behind "nuts"... 1) source of pleasure, or 2) wild west poker players would put their wagon axle nuts on the table when they wanted to prove that their bet was good b/c they would not be able to drive away if they lost (and would then have to pay their debt).  If you like the wild west version, then maybe you might also think that people would only play "the nuts" if nuts were always unbreakable, if tie-able (thus, nuts = you can't break this, stone cold = you can't even tie this). So, can your nuts be broken?

Royal flush = stone cold nuts (assorted, all kinds of nuts).