A yammo can have a few different definitions, depending on context. In general, it is used to refer to a person who is or is acting like an idiot, fool, moron, dimwit, simpleton, tool, lunk head, asshole, jerk, etc.
An explanation of a brutal dunk in basketball that involves the dunker to drag their nuts across the face of the person being dunked on. Usually an unexpected dunk to finish off a game or to outright disrespect the opponent.
“Did you see Etty and Neo play on the court last Friday?”
“Yeah Neo brought the game to 21-3, she got yammed on”