bike

One explanation for the form with /k/ is that bicycle was parsed to bi(cy)c(le). An alternative explanation is that it was parsed to bic(ycle) but since speakers are aware of a general /k/~/s/ alternation (as in electric ~ electricity etc.), the softened /s/ was restored to a default /k/ when the “ending” -ycle was dropped. Similar cases are merc /mɜɹk/, spec /spɛk/ for mercenary, specify. It seems unlikely, however, that this process is purely phonological and not at least partially based on the spelling ⟨c⟩.

From Middle English bike, byke (“a nest of wild bees or wasps; also, honeycomb”), of unknown origin. Perhaps a back-formation of Middle English *bykere (“beekeeper”), from Old English bēocere (“beekeeper”); or from Old English *bȳc, a byform of Old English būc (“belly; vessel; container”). Compare also Scots byke (“beehive, anthill; home, dwelling”), Old Norse bý (“bee”).