'moony-eyed' is an English compound formed from 'moony' + 'eyed'. 'Moony' comes from 'moon' with the adjectival suffix '-y'; 'moon' in Old English was 'mōna' (Old English).

'moon' (Old English 'mōna') passed into Middle English as 'mone'/'mone', later 'moon'. The adjective 'moony' developed by adding the suffix '-y' to mean 'relating to or affected by the moon' or 'dreamy', and compounds like 'moony-eyed' emerged in modern English usage (recorded from the 19th century onward) to describe a dreamy or lovestruck gaze.

Initially related to the moon (and by extension to lunacy or moonlike qualities), the compound evolved to mean 'dreamy, romantically naive, or lovestruck' rather than a literal relation to the moon.