means a brownish coloration the skin develops after exposure to sunlight, or the light yellowish-brown color itself.

from From Old English 'tannian,' to convert hide into leather — a verb borrowed from Medieval Latin 'tannare,' which leaned on 'tannum,' crushed oak bark, the bitter stuff that cured the skins. Oak bark stained leather a warm brown, so 'tan' became the color's name. The skin sense — sun browning your hide like a tanner browns a cowhide — is the same image turned on yourself. The bark word may trace back further to a Celtic root for oak, though that thread frays the deeper you pull it.