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1.
adj. slow, stupid, mentally absent and at the same time maintaining a positive attitude. Refers to the character Caboose in the web show Red vs Blue.
Comparative: Caboosier
Superlative: Caboosiest
Church: This is Caboose's mind, so everybody here is pretty much as Caboosey as the world. That means everyone here is going to be either really happy or make no sense whatsoever.

Bob: I thought the meeting was tomorrow.
Josh: They sent like twenty emails saying it was yesterday. You may be the Caboosiest person I have ever met.
by Agent Washingtub December 20, 2008