'infrequently-employed' is a Modern English compound formed by combining the adverb 'infrequently' (from 'in-' + 'frequent', ultimately from Latin roots related to 'frequens') and the past participle 'employed' (from Old French 'employer'/'emploir', meaning 'to use or put to work').

'infrequently' developed in Middle English from Old French and Latin elements ('in-' + 'frequens'), while 'employ' entered English via Old French 'employer' and Middle English forms before becoming the modern verb 'employ' and its past participle 'employed'. The compound itself is a later Modern English formation using these established elements.

Individually, 'infrequent' originally meant 'not occurring often' and 'employ' meant 'to put into use'; together in Modern English they formed the descriptive phrase 'infrequently-employed', meaning 'seldom used', a meaning that follows directly from the constituent parts.