early 14c., "structure built against a wall to give it stability," from Old French (arc) botrez "flying buttress," apparently from bouter, boter "to thrust against," a word of Frankish origin (compare Old Norse bauta "to strike, beat"), from Proto-Germanic *butan, from PIE root *bhau- "to strike." Figurative sense "any source of support" is from mid-15c.
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