Source of some base humor, for short?

USO

Think of a wartime organization that might be abbreviated in jokes.

The clue plays on the double meaning of "base." In military slang, "base" can refer to a USO (United Service Organizations) facility, known for entertaining troops. The word "base" also hints at lowbrow or crude humor, tying into the "humor" part of the clue. The abbreviation "USO" fits as a short answer for this wordplay.