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1.
when someone accidentally sticks a finger in their partner's butthole in the heat of the moment.
Did you hear about Lydia? Yeah, Paul's digits slipped and he gave her a good butterfingering.
by jessas December 24, 2012
 
2.
When a women is finger-banged and butter is used as the lubricant. In Australia this may also involve eating the butter off a piece of toast.
"I was butter fingering you after class, It didn't mean we were dating!"
by HankyPanky420 February 24, 2009