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.
That guy
almost cut me off! Fucking yammo!
I can't
believe he didn't know her name was Mulva. What a yammo.
Don't be such a yammo - VOTE.