WordNet (r) 3.0 (2006):

hard hat
    n 1: a worker skilled in building offices or dwellings etc.
         [syn: construction worker, hard hat]
    2: a lightweight protective helmet (plastic or metal) worn by
       construction workers [syn: hard hat, tin hat, safety
       hat]

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "hard hat":
   Bircher, Bourbon, Tory, aegis, arm guard, backstop, buffer,
   bulwark, bumper, chauvin, chauvinist, conservatist, conservative,
   contraceptive, copyright, crash helmet, cushion, dashboard,
   diehard, dodger, extreme right-winger, face mask, fender,
   finger guard, flag waver, foot guard, fuse, goggles, governor,
   guard, guardrail, hand guard, handrail, helmet,
   hundred-percent American, hundred-percenter, imperialist,
   insulation, interlock, jingo, jingoist, knee guard, knuckle guard,
   laminated glass, life preserver, lifeline, lightning conductor,
   lightning rod, mask, monarchist, mudguard, nationalist, nose guard,
   pad, padding, palladium, patent, patriot, patrioteer, pilot,
   preventive, prophylactic, protective clothing, protective umbrella,
   radical right, reactionarist, reactionary, reactionist, right,
   right wing, right-winger, rightist, royalist, safeguard, safety,
   safety glass, safety plug, safety rail, safety shoes,
   safety switch, safety valve, screen, seat belt, shield, shin guard,
   social Darwinist, standpat, standpatter, sun helmet, superpatriot,
   ultraconservative, ultranationalist, umbrella, windscreen,
   windshield