Old English tacen "sign, symbol, evidence" (related to tæcan "show, explain, teach"), from Proto-Germanic *taiknan (cf. Old Saxon tekan, Old Norse teikn "zodiac sign, omen, token," Old Frisian, Middle Dutch teken, Dutch teken, Old High German zeihhan, German zeichen, Gothic taikn "sign, token"), from PIE root *deik- "to show" (see teach).
Meaning "coin-like piece of stamped metal" is first recorded 1590s. Original sense of "evidence" is retained in by the same token (mid-15c.), originally "introducing a corroborating evidence."