tetchy (adj.)

Old English tacen "sign, symbol, evidence, portent" (related to verb tæcan "show, explain, teach"), from Proto-Germanic *taikna- (source also of Old Saxon tekan, Old Norse teikn "zodiac sign, omen, token," Old Frisian tekan, Middle Dutch teken, Dutch teken, Old High German zeihhan, German zeichen, Gothic taikn "sign, token"). According to Watkins, this is from PIE root *deik- "to show," also "pronounce solemnly." Compare, from the same root, German zeigen "to show," Old English teon "to accuse,"

Also in late Old English as "observable characteristic or action indicating an inner state; means of identifying a person." By c. 1200 of physical objects representing an action, state, deity, etc.; "keepsake, that which serves as a reminder." From late 14c. as "act performed in recognition of a contract or agreement; public, symbolic display or act." The meaning "coin-like piece of stamped metal" is by 1590s.

The Middle English sense of "evidence, support for a belief" is retained in by the same token (mid-15c.), a phrase used in introducing a corroborative circumstance, according to Century Dictionary "almost equivalent to 'this in testimony.' "

"slightly mad, a little crazy," 1930, U.S. colloquial variant of touched in the sense of "emotionally stirred." Also compare tetchy.