tan [OE] The original sense of tan is to convert skins into leather. The sense of the colour that the skin acquires after exposure to the sun dates only to the middle of the 18th century, although the verb in this sense is mid 16th. Tan probably comes directly from Latin tannare, but may ultimately go back to a Celtic word for an oak tree. This reflects the process of tanning, whereby the crushed bark of an oak was steeped in water in which skins and hides were then immersed. Oak bark was used because it is rich in tannins [E19th], compounds which will tan. The related word tawny [ME] comes from Old French tauné, ‘tanned’.