From OE 'hætt,' from PIE *kadh- (to shelter, cover) — etymologically a portable shelter for the head.
A shaped covering for the head, typically having a crown and brim.
From Old English 'hætt' (hat, head covering), from Proto-Germanic *hattuz, from PIE *kadʰ- ('to cover, to protect, to shelter'). The PIE root connects head covering to the broader concept of protection: Latin 'cassis' (helmet) may be related, and the root sense of 'shelter' links clothing to architecture. The Proto-Germanic form was specific to a head covering with a brim or defined shape, distinguishing
The phrase 'mad as a hatter' predates Lewis Carroll's Alice in Wonderland by decades. It referred to real occupational illness: hat-makers in the 18th and 19th centuries used mercury nitrate to cure felt, and chronic mercury exposure caused tremors, mood swings, and psychosis — a condition called 'erethism' or 'mad hatter disease.'