'unforgettably' originates from the word 'unforgettable', which is derived from the prefix 'un-' meaning 'not', and the root 'forget', which comes from Old English 'forgietan', meaning 'to lose remembrance'.

'unforgettably' evolved from the adjective 'unforgettable', which was formed by adding the adverbial suffix '-ly' to describe the manner of being unforgettable.

Initially, it meant 'in a manner that cannot be forgotten', and this meaning has remained consistent in modern usage.