the.com/hard hat

means A rigid protective helmet worn on construction and industrial sites to shield the head from falling objects and impacts.

from The term shows up in English in the 1930s, naming the stiff protective headgear being adopted on construction and industrial job sites — early versions were often boiled or resin-treated canvas, later steel, then molded plastic. 'Hard hat' stuck as a literal description (hard, as opposed to a soft cap) and by the mid-20th century had also slid into slang for construction workers themselves — and even, during 1960s labor politics, for a certain blue-collar conservative stance, simply because that's who wore them.