the word 'tan' entered english from a celtic language, likely old cornish or old breton, where a word like tann referred to the oak tree or its bark. in late old english, it specifically denoted the crushed bark used in the process of turning animal hides into leather. the verb sense, "to convert hide into leather," naturally extended to describe the darkening or browning effect of that process. by the 16th century, this association with colour change was applied metaphorically to the browning of human skin by the sun, which became the dominant modern meaning, while the original leather-working sense became specialized.

this word is a simplex form. it is a borrowing from a celtic source (likely old cornish or old breton) into late old english, where it originally referred to the crushed bark of an oak tree used for converting hides into leather. the modern spelling and pronunciation are direct continuations of this form.