'unhealthy-looking' originates from Modern English, formed by the negative prefix 'un-' (from Old English 'un-' meaning 'not'), the adjective 'healthy' (from Old English 'hǣlþ'/'hælu' meaning 'wholeness, health'), and the present-participial adjective 'looking' (from the verb 'look', Old English 'lōcian'/'lōcian' meaning 'to see/appear').

'unhealthy-looking' developed by compounding: 'healthy' (from Old English 'hǣlþ' → Middle English 'halthi') became 'unhealthy' with the prefix 'un-' in Middle/Late Middle English; 'look' (Old English 'lōcian' → Middle English 'loken') produced the participle 'looking'. These elements were combined in Modern English into the adjectival phrase 'unhealthy-looking'.

The compound originally and historically described the absence of health in appearance ('not healthy in look'), and its meaning has remained essentially the same: 'appearing to be unhealthy'.