Top Definition
The unknown and rather disturbing substance that accumulates under your nails following scratching an itchy ass; a less dense form of ass cheese.

Also, a reference to something nasty;
"DUDE! Quit digging in your ass! Now you have rectal butter under your nails!"

"That chick needs to take a damn shower! She smells like rectal butter!"
by m_4_reall March 12, 2010

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