WordNet sense 1 (someone who learned to fight in the streets rather than being formally trained in the sport of boxing):

street fighter

WordNet sense 2 (an aggressive and violent young criminal):

goon, hood, hoodlum, punk, thug, toughie, strong-armer

WordNet sense 3 (a cruel and brutal fellow):

bully, hooligan, roughneck, rowdy, ruffian, yob, yobbo, yobo

WordNet sense 4 (violent and lawless):

ruffianly

WordNet sense 5 (acting with or marked by or resulting from great force or energy or emotional intensity):

violent

WordNet sense 6 (tough and callous by virtue of experience):

pugnacious

WordNet sense 7 (made tough by habitual exposure):

hardened

WordNet sense 8 (having calluses; having skin made tough and thick through wear):

callous

WordNet sense 9 (physically toughened):

toughened

WordNet sense 10 (having undesirable or negative qualities):

bad

WordNet sense 11 (making great mental demands; hard to comprehend or solve or believe):

baffling, knotty, problematic, problematical

WordNet sense 12 (not easy; requiring great physical or mental effort to accomplish or comprehend or endure):

difficult, hard

WordNet sense 13 (substantially made or constructed):

sturdy

WordNet sense 14 (sturdy and strong in constitution or construction; enduring):

rugged, rugged

WordNet sense 15 (providing or experiencing physical discomfort):

uncomfortable