The term puña literally refers to an assasin's dagger; the type of daggar assisins would have used in milenia gone by to back-stab a victem to death. Usually these weapons were poison tipped to ensure that they got the job done. Puñal is an adjective derived from puña. Something that is puñal has the quality of an assasin's poisoned daggar. The word is sometimes misused, by non-Spanish speaking Mexicans, as a noun.
Metaphorically, the term refers to a sneaky homosexual male who you can't turn your back on because he has a queer eye for straight guys. He may "back stab you with his poison tipped daggar" if you turn your back on him, or bend over for soap. It's a funy combo of clashing imagery.
The term could be used to describe the qualities of a jailhouse rapist who will nail you if you bend over for soap in the shower.
Piñche mayate puñal!
You wretched nigger faggot rapist!
Piñal marica puñal!
You wretched little sissy faggot rapist!
Piñal puto puñal!
You rotten wreched jail house boy faggot rapist