It's clear that the word references the straightforward non plus meaning "no more". You've come to a dead end; you don't know what to think or do. It might act

It's clear that the word references the straightforward non plus meaning "no more". You've come to a dead end; you don't know what to think or do. It might actually be from non plus ultra: nothing more beyond. When you've reached the non plus ultra point in your journey, your ass has been nonplussed.