tetchy adj.

Old English tacen "sign, symbol, evidence" (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"), from PIE root *deik- "to show," also "pronounce solemnly."

Meaning "coin-like piece of stamped metal" is first recorded 1590s. Older sense of "evidence" is retained in by the same token (mid-15c.), originally "introducing a corroborating circumstance" [OED].

Proto-Indo-European root meaning "to show," also "pronounce solemnly," "also in derivatives referring to the directing of words or objects" [Watkins].

It forms all or part of: abdicate; abdication; addict; adjudge; apodictic; avenge; benediction; betoken; condition; contradict; contradiction; dedicate; deictic; deixis; dictate; diction; dictionary; dictum; digit; disk; ditto; ditty; edict; Eurydice; index; indicate; indication; indict; indiction; indictive; indite; interdict; judge; judicial; juridical; jurisdiction; malediction; malison; paradigm; policy (n.2) "written insurance agreement;" preach; predicament; predicate; predict; prejudice; revenge; soi-disant; syndic; teach; tetchy; theodicy; toe; token; valediction; vendetta; verdict; veridical; vindicate; vindication; voir dire.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dic- "point out, show;" Greek deiknynai "to show, to prove," dike "custom, usage;" Latin dicere "speak, tell, say," digitus "finger," Old High German zeigon, German zeigen "to show," Old English teon "to accuse," tæcan "to teach."

"apt to take offense at slight provocation," c. 1600, perhaps an alteration of tetchy (q.v.) influenced by touch (v.). Related: Touchiness.