hard hat
Word family verbhardenadjectivehardenedhardishhard-and-fasthardbackedhard-bittenhard-boiledhard-earnedhard-foughthard-headedhard-heartedhard-hittinghardlinehardmouthedhardnosedhard-pressedhardshellhard-wearinghard-wiredhard-wonhardworkinghardnounhardenerhardeninghardnesshardshiphardbackhardbakehardballhardbeamhardboardhard copyhard corehard courthard currencyhard diskhard drivehardfacehardgrasshardhackhard hathardheadhard labourhard linehardlinerhard-luck storyhardpartshard rockhard sellhard shoulderhardtackhardtailhardtophardwarehardwaremanhardwood
Construction
ˌhard ˈhat noun [countableC] 
TBCa protective hat, worn especially by workers in places where buildings are being built 〔尤指建筑工人戴的〕安全帽，防护帽 →5 see picture at 见图 hatExamples from the Corpus
hard hat• If hydrogen sulfide or some other poisonous gas is detected, Donahue dons an airtight breathing device and a hard hat.• A hard hat can protect you from having a metal spike go through your skull and into your brain.• I had to put my hard hat on and go to work.• When no vapors are present, he simply wears the hard hat.
From Longman Business Dictionary
hard hatˈhard hat noun [countableC]
1a protective hat worn by builders
Hard hats are now required by the Occupational Safety and Health Administration for all construction workers.
2American EnglishAmE informalJOB a worker in the building industry
People working in television tend not to feel at home with hard hats and blue collars.