Asshole was sucking buttermilk

Can be a noun and verb bass upon context.
The act of your asshole uncontrollably quivering to hold back an explosive, juicy, wet, diarrhea-filled colon. The reason it is quivering is usually from extreme beer drinking and not wanting to hit the bathroom until the last minute. Or, when you first get up in the morning and you just have to hold the quivering back until you just can’t wait any longer.

Man, that chick at the party was hot. We had several rounds of beer and our conversations we going great but I had to break away to hit the bathroom because my asshole was sucking buttermilk and I just could not hold it any longer!