'bactericidal' originates from New Latin, specifically the word 'bactericidalis', where 'bacteri-' (from Greek 'bakterion') meant 'little rod' (used for 'bacterium') and '-cidal' (from Latin '-cidus'/'-cida') meant 'killing'.

'bactericidally' changed from the adjective 'bactericidal' by adding the adverbial suffix '-ly' in Modern English; 'bactericidal' itself was formed in scientific New Latin from Greek 'bakterion' -> New Latin 'bacterium' + Latin-derived '-cidal'.

Initially it meant 'capable of killing bacteria', and over time it has kept this meaning; the adverbial form simply denotes doing something in that manner ('in a way that kills bacteria').