'seldom-employed' is a compound of 'seldom' and 'employed'. 'seldom' originates from Old English, specifically the word 'seldum', where 'seld-' meant 'rare' or 'infrequent'. 'employed' (past participle of 'employ') originates from Old French, specifically the word 'employer', where the root meant 'to use' or 'to engage'.

'seldom' changed from Old English 'seldum' to Middle English forms such as 'seldom' and eventually became the modern English 'seldom'. 'employer/emploir' in Old French developed into Middle English 'employen' and its past participle forms, leading to modern English 'employ' and 'employed'.

Initially, the elements meant 'rarely' (for 'seldom') and 'to use or hire' (for 'employ'); combined, the compound came to mean 'rarely used or hired', a meaning that matches its current usage.