"metal vessel used for boiling or heating liquids over a flame," Old English cetil, citel (Mercian), from Proto-Germanic *katilaz (compare Old Saxon ketel, Old Frisian zetel, Middle Dutch ketel, Old High German kezzil, German Kessel), which usually is said to be derived from Latin catillus "deep pan or dish for cooking," a diminutive of catinus "deep vessel, bowl, dish, pot," from Proto-Italic *katino-.
This word has been connected with Greek forms such as [kotylē] "bowl, dish." Yet the Greek word is no perfect formal match, and words for types of vessels are very often loanwords. It seems best to assume this for catinus too. [de Vaan]
One of the few Latin loan-words in Proto-Germanic, along with *punda- "measure of weight or money" (see pound (n.1)) and a word relating to "merchant" that yielded cheap (adj.). "[I]t is striking that all have something to do with trade" [Don Ringe, "From Proto-Indo-European to Proto-Germanic," Oxford 2006]. Perhaps the Latin word was confused with a native Germanic one.
Spelling with a -k- (c. 1300) probably is from influence of Old Norse cognate ketill. The smaller sense of "tea-kettle" is attested by 1769.
Kettle of fish "complicated and bungled affair" (1715), sometimes is said to be from a Scottish custom of a kettle full of fish cooked al fresco at a boating party or picnic, but this custom is not attested by that phrase until 1790. Perhaps it is rather a variant of kittle/kiddle "weir or fence with nets set in rivers or along seacoasts for catching fish" (c. 1200, in the Magna Charta as Anglo-Latin kidellus), from Old French quidel, probably from Breton kidel "a net at the mouth of a stream."
Kettle was used in geology for "deep circular hollow in a river bed or other eroded area, pothole" (1866), hence kettle moraine (1883), one characterized by such features.