In the comments to the accepted answer to this Unix & Linux StackExchange question, it is stated that $_ is one of only some 7 special parameters defined by POSIX.  I can't find that definition although I can find eight other special parameters defined by POSIX and I've seen $_ defined in the ksh and bash man pages.  Can someone point me to the definition in POSIX?
Where is $_ defined by POSIX?
bash;shell;ksh;posix
The only reference I can find to the special parameter $_ in POSIX is in the rationale section on Shell Variables. This excerpt implies that it was used by a variety of shells, but not in a standard way by all and was omitted intentionally:_(Underscore.) While underscore is historical practice, its overloaded usage in the  KornShell is confusing, and it has been omitted from the Shell and Utilities volume  of IEEE Std 1003.1-2001.So it may be used by most or all POSIX-conforming shells, but it is decidedly not defined by POSIX.