John Simpson shared some of these examples with Kathryn Ryan, but my favourite has to be the verb form of the noun 'inkling' - 'to inkle'. This came into the English language in Medieval times - its first use is recorded as around 1150, with the meaning 'to utter or communicate in an undertone, or to whisper or hint'. While today we have kept the noun in operation, we no longer use the verb form. It is what is known in the trade as a 'lost verb'.
It seems a shame. Perhaps we could find space for this charming word in everyday language again, by swapping out a useless word or phrase, such as the much over-used and usually utterly redundant 'going forward'?