Definition of butt-in:
(n) :
(countable) The larger or thicker end of something; the blunt end, in distinction from the sharp or narrow end
(n) :
(Canada, US, Cumbria, Philippines, slang) The buttocks or anus (used as a minced oath in idiomatic expressions; less objectionable than arse/ass).
(n) :
(slang) The whole buttocks and pelvic region that includes one's private parts.
(n) :
(slang, metonymic) Body; self.
(n) :
(leather trades) The thickest and stoutest part of tanned oxhides, used for soles of boots, harness, trunks.
(n) :
(countable) The waste end of anything.