The word clock has quite a long history, unsurprising for such a common and simple word.
It probably came directly to English from the Middle Dutch clocke, which in turn is probably derived from the Old North French word cloque, meaning bell. This word probably comes from the Medieval Latin clocca, also meaning bell, which itself was more than likely derived from a Celtic word, either the Irish clocc or Welsh cloch.
There aren’t many English words derived from Celtic languages, unsurprisingly, considering the relatively small numbers of speakers of these languages throughout history, and the isolation of the places they’ve seen spoken.
In this case it’s not too surprising though. In the first millennium AD, Irish monasteries became important centres of learning and preservation of knowledge, earning the island the nickname The Land of Saints and Scholars. It’s quite probable that in Irish monasteries, a bell (clocc) was rung to indicate meal and prayer times. This word then spread across the rest of Europe, possibly helped by missionaries, eventually leading to the modern English word clock.
As a sidenote, I’ve long wondered whether there might be some link between the Irish word cloch (meaning stone, sounds like cluck if you clear your throat right at the end) and clocc (clog in modern Irish). There may not be a link, unless the monks used to strike a rock or stone wall before they got nice metal bells.
One word that certainly is distantly related to the word clock is cloak. The word has its origins in the Old French dialectical word cloke, a variant on the Old (and modern) French word for bell: cloche. The word came about because of the bell-like shape of the garment.
Sometimes words look and sound similar simply through coincidence. Occasionally though, a little digging reveals that words like clock and cloak are kinds of linguistic cousins, with a bell being the far-from-obvious link between the two, joining them on their journey across Europe and through the centuries.