Old English brysan "to crush, pound, injure by a blow which discolors the skin," from Proto-Germanic *brusjan, from PIE root *bhreu- "to smash, cut, break up" (source also of Old Irish bronnaim "I wrong, I hurt;" Breton brezel "war," Vulgar Latin *brisare "to break"). Merged by 17c. with Anglo-French bruiser "to break, smash," from Old French bruisier "to break, shatter," perhaps from Gaulish *brus-, from the same PIE root. Of fruits from early 14c. Intransitive sense "become bruised" is by 1912. Related: Bruised; bruising.
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Definitions of bruiser from WordNet
a large and strong and heavyset man;
a thick-skinned bruiser ready to give as good as he got