'glassy-eyed' is a compound formed from 'glassy' + 'eyed'. 'Glassy' comes from 'glass' + the adjective-forming suffix '-y'; 'glass' originates from Old English 'glæs' (meaning 'glass'). 'Eyed' is formed from 'eye' + the adjectival suffix '-ed' meaning 'having the specified kind of eyes'.

'glass' originates from Old English 'glæs', from Proto-Germanic '*glasaz' (related to words for 'glass' or 'amber'); 'glassy' developed in Middle English by adding '-y' to produce an adjective meaning 'resembling or made of glass'. 'Eye' originates from Old English 'ēage' (Proto-Germanic '*augô', from PIE root '*okw-' meaning 'to see'). The compound 'glassy-eyed' arose in modern English by combining these elements to describe a particular appearance of the eyes.

Originally the components referred literally to 'eyes like glass' (shining or glass-like); over time the compound came to describe both the literal glazed or watery look and a figurative vacant or unseeing expression often linked to physiological or emotional states.