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.