Not wearing a hat; without a hat on one's head.
Formed from 'hat' (Old English 'hæt') with the suffix '-less' (meaning without, from Old English). This straightforward compound has been used since at least the 16th century.
Before the 20th century, appearing hatless in public was considered scandalous, especially for women—hats were so essential to proper dress that going without one marked you as either poor, radical, or immoral. The shift to hatless fashion happened astonishingly fast during the 1960s.
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