'regularly-used' is a compound of two elements. 'regularly' originates from Latin via Old French, specifically the adjective 'regularis', where 'regula' meant 'rule'. 'used' originates from Latin 'uti' through Old French 'user', where 'uti' meant 'to use'.

'regularis' developed into Old French 'regulier' and Middle English 'regular', with the adverb formed by adding the suffix '-ly' to make 'regularly'. Separately, Latin 'uti' produced Old French 'user' and Middle English 'use', whose past participle became 'used'. The modern compound combines these developments as 'regularly used'.

Initially, 'regularly' meant 'in a manner conforming to a rule' and 'used' meant 'employed or utilized'; together the compound evolved to mean 'employed frequently' or 'used on a regular basis'.