late Old English tannian "to convert hide into leather" (by steeping it in tannin), from Medieval Latin tannare "tan, dye a tawny color" (c. 900), from tannum "crushed oak bark," used in tanning leather, probably from a Celtic source (such as Breton tann "oak tree"). The meaning "make brown by exposure to the sun" (as tanning does to hides) first recorded 1520s; intransitive sense also from 1520s. Of persons, not considered an attractive feature until 20c.; in Shakespeare, "to deprive of the freshness and beauty of youth" (Sonnet CXV). As an adjective from 1620s. To tan (someone's) hide in the figurative sense is from 1660s. Related: Tanned; tanning. German Tanne "fir tree" (as in Tannenbaum) might be a transferred meaning from the same Celtic source.
"bronze color imparted to skin by exposure to sun," 1749, see tan (v.). Earlier as "substance made of crushed bark used in making leather" (c. 1600). As a simple name for a brownish color, in any context, it is recorded from 1888. The adjective meaning "of the color of tanned leather" is recorded from 1660s. Tan-line attested from 1979.