Real Dictionary

boomerang
countable noun: A boomerang is a curved piece of wood which comes back to you if you throw it in the correct way. Boomerangs were first used by the people who were living in Australia when Europeans arrived there.
verb: If a plan boomerangs, its result is not the one that was intended and is harmful to the person who made the plan.

boomerang in American English
a flat, curved stick that can be thrown so that it will return to a point near the thrower: it is used as a weapon by Australian Aborigines
to act as a boomerang; result in harm to the originator
intransitive verb: to come back or return, as a boomerang
noun: a bent or curved piece of tough wood used by the Australian Aborigines as a throwing club, one form of which can be thrown so as to return to the thrower

boomerang in British English
noun: a curved flat wooden missile of native Australians, which can be made to return to the thrower
verb: to recoil or return unexpectedly, causing harm to its originator; backfire