'validly' originates from English, formed by adding the adverbial suffix '-ly' to the adjective 'valid'. 'valid' itself originates from Latin, specifically the word 'validus', where the root 'val-' (from Latin 'valēre') meant 'to be strong, to be worth'.

'valid' entered English via Old French/Latin influence from Latin 'validus'. Over time Middle English adopted the form 'valid', and the productive English adverbial suffix '-ly' (from Old English '-līc'/'-lic') was added to produce 'validly'.

Initially related to physical strength or worth ('strong, powerful'), the sense shifted toward 'having force or efficacy'; in modern usage it denotes legal, logical, or formal acceptability—hence 'validly' = 'in a valid way'.