COAT is an acronym for "Come on All Things" or "Cum on all things." It is a versatile meme that can be used to express hope, admonishment, or pleasure.
COAT evolved from another pseudo-meme that pertained to the use the phrase "Come on, (insert Padre player name)," which is used as a form of encouragement. Frequently, the comma would accidentally (or intentionally) be left out of the sentence, which would change the meaning from an innocent cheer to an explicit sexual act. The typical response when the comma was left out would simply be: Comma.
The phrase was delightfully filthy and has since become part of the GLB vernacular.