Real Dictionary

boxer
countable noun: A boxer is a dog with short hair and a rather flat face.
countable noun: A boxer is someone who takes part in the sport of boxing.
plural noun: Boxers are the same as boxer shorts.

Boxer in American English
a member of a Chinese society that led an unsuccessful uprising (the Boxer Rebellion, 1900) against foreign powers and foreigners in China, as a result of which China was forced to make economic and territorial concessions
one who boxes; pugilist; prizefighter; often, one skilled in the offensive and defensive tactics of boxing, as distinguished from a slugger
noun: a member of a Chinese secret society that carried on an unsuccessful uprising, 1898–1900 (Boxer Rebellion), principally against foreigners, culminating in a siege of foreign legations in Peking that was put down by an international expeditionary force
noun: a person who fights as a sport, usually with gloved fists, according to set rules; prize-fighter; pugilist

boxer in British English
noun: a member of a nationalistic Chinese secret society that led an unsuccessful rebellion in 1900 against foreign interests in China
noun: a person who boxes, either professionally or as a hobby; pugilist