'frequently-employed' is a modern English compound formed from 'frequently' and 'employed'. 'frequently' ultimately originates from Latin, specifically the word 'frequēns' (via Old French/Middle English), where the root 'frequ-' meant 'crowded, repeated'. 'employed' originates from Old French, specifically the word 'employer', where the prefix 'en-' meant 'in/into' and the root related to Latin 'plicare' meant 'to fold' (extended figuratively to mean 'to use' or 'to apply').

'frequently-employed' developed from the phrase 'frequently employed' in Early Modern English; the elements come from Middle English/Old French roots ('frequent' from Latin 'frequēns' and 'employ' from Old French 'employer') and were later combined into the modern compound 'frequently-employed'.

Initially the component words meant 'often' (frequently) and 'to use/hire' (employ). Over time the combined phrase retained the basic sense of 'often used' or 'often hired', with little shift in core meaning.