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.
yammo: define #2
A prostitute’s gold tooth, according to a few I’ve heard. Not sure if it is really a widespread term.
“Look at the Yammo on that fuckin prostitute! She needs to go to the dentist”