A shortened version of the word mental or mentally challenged. It is used as a derogatory term to state that someone is acting in a mental fashion. The root "men" deriving from the Latin meaning "to think" gives the base of the word and adding "to" that means nothing would make it" to think not at all" thus creating the word 'mento'.
person 1: Geez, do you see that person over there?
person 2: Yeah, they're sure acting like a mento.
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