'faithful' originates from Old English and Middle English elements: the noun 'faith' (from Old French 'feid/feith', ultimately from Latin 'fides' meaning 'trust, belief') combined with the adjective-forming suffix '-ful' (from Old English 'ful' meaning 'full of').

'faithful' developed in Middle English as 'feithful' or 'faithful' formed from Old French 'feid/feith' (for 'faith') plus Old English/Old Norse-influenced suffix 'ful', and eventually became the modern English word 'faithful'.

Initially it meant 'full of faith' or 'having faith/trust'; over time this broadened to the current sense of 'loyal, devoted, or steadfast'.