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.
Nickname given to bales of marijuana thrown overboard or out of airplanes. Origin is reportedly from members of the USCoast Guard in the area of Southern Florida.
"Base we've got some square groupers floating outside of Miami harbor."