When a man covers himself in "I cant believe its not butter" runs on a tile floor and rams his dick into the female. Much like a billy goat would ran its horns,
The phrase "baptising a billy goat" is a phrase used mostly in the Southeast part of the United States. It is used to describe a useless activity that will produce no meaningful results. It is often concluded with the phrase "It's trouble for nothing."
John got caught red-handed with his bestfriend's girlfriend. Trying to convince his friend that he didn't mean anything by it was like "baptising a billy goat. It was trouble for nothing.
Or
Trying to get Dick Cheney to join the ACLU would be like baptising a billy goat. It's trouble for nothing.
When a super hot chick bites the longest part of your beard/goatee and gives it a little tug and shake.
Roy: "She billy-goated me"
Brian: "She just clapped on and shocked you like a rag-doll?"
Roy: "Yeah dude, weirdest and sexiest thing ever. You need to grow yours out."