late 15c., "process of tanning leather," verbal noun from tan (v.). Intransitive sense "process of getting suntan" is from 1944. Tanning booth is attested by 1978; tanning bed by 1981.
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.