Word Origin & History
tan (v.)
late O.E. tannian "to convert hide into leather" (by steeping it in tannin), from M.L. tannare "tan, dye, a tawny color" (c.900), from tannum "crushed oak bark," used in tanning leather, probably from a Celtic source (e.g. Breton tann "oak tree"). The meaning "make brown by exposure to the sun" first recorded 1530. To tan (someone's) hide in the figurative sense is from 1670. The adj. tan "of the color of tanned leather" is recorded from 1665; the noun sense of "bronze color imparted to skin by exposure to sun" is from 1749; as a simple name for a brownish color, in any context, it is recorded from 1888.
[tan] 
verb, tanned, tan⋅ning, noun, adjective, tan⋅ner, tan⋅nest.