Short for "blackout," as in drinking entirely too much alcohol, sometimes combined with other substances. Can be used as a verb, a noun, or an adjective. On a scale of 1-10, B.O. stands for all numbers 11 and higher, and is almost always associated with belligerence and depravity. Much like alcohol itself, the term is used heavily in Wisconsin.
Verb: Let's get Rumplemintz and B.O. tonight!
Noun: That kid is a walking B.O. every weekend.
Adjective: I was completely B.O. last night, don't remember anything.