'tar-based' is a compound of 'tar' and 'based'. 'tar' originates from Old English 'teoru' (meaning 'pitch, tar'), and 'base' (used here as 'based') ultimately comes from Latin 'basis' via Old French 'bas', where 'basis'/'bas' meant 'foundation' or 'low part'.

'tar' existed in Old English as 'teoru' and referred to pitch or tar; the adjectival use of past-participial forms like 'based' (from verb 'base') developed into a productive combining form (X-based) in modern English, producing compounds such as 'water-based', 'oil-based', and later 'tar-based'.

Initially 'tar' referred specifically to natural pitch or coal tar; over time the compound 'tar-based' has come to mean anything whose primary ingredient or foundation is tar (including manufactured sealants, coatings, and other products).