'heat-absorbing' is a compound of the noun 'heat' and the present participle 'absorbing.' 'Heat' goes back to Old English 'hǣtu' meaning 'warmth,' and 'absorb' originates from Latin 'absorbere' meaning 'to suck in' or 'draw in.'

'absorb' entered English via Old French 'absorber' and Middle English; over time the present participle 'absorbing' was used adjectivally and was combined with 'heat' to form the modern compound adjective 'heat-absorbing.'

Originally the components described the action 'to take in heat'; the compound later became established as an adjective meaning 'having the property of absorbing heat.'