butt (v.)

"hit with the head, strike by thrusting" (as with the end of a beam or thick stick), c. 1200, from Anglo-French buter, Old French boter "to push, shove, knock; to thrust against," from Frankish or another Germanic source (compare Old Norse bauta, Low German boten "to strike, beat"), from Proto-Germanic *butan, from PIE root *bhau- "to strike."

Meaning "to join at the end, be contiguous" is from 1660s, partly a shortening of abut. To butt in "rudely intrude" is American English slang, attested from 1900. Related: Butted; butting.