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.