Origins
The word 'cheese' is one of the oldest Latin loanwords in English, borrowed so early that it predates the English language's arrival in Britain. Old English 'cēse' (also spelled 'cȳse,' 'cēase') derives from West Germanic *kāsī, which was borrowed from Latin 'cāseus' (cheese) during the period when Germanic tribes were in direct contact with the Roman Empire — roughly the first through fifth centuries CE. The borrowing occurred on the European continent, before the Angles, Saxons, and Jutes migrated to Britain in the fifth and sixth centuries. When they arrived, they brought the word with them.
The early date of the borrowing is confirmed by the word's distribution across the West Germanic languages: German 'Käse,' Dutch 'kaas,' and Old Frisian 'tzīse' all descend from the same West Germanic form *kāsī, indicating that the borrowing occurred before these languages fully diverged. The sound changes are regular: Latin 'cā-' became West Germanic *kā-, and the subsequent palatalization of initial /k/ before a front vowel in Old English produced the /tʃ/ (ch-sound) of modern 'cheese.'
Latin 'cāseus' is of uncertain further origin. Some etymologists have connected it to PIE *kwat- (to ferment, to become sour), which would make cheese literally 'the fermented thing.' Others have proposed a connection to a pre-Indo-European substrate language, since cheese-making in the Mediterranean predates the arrival of Indo-European speakers. The uncertainty is characteristic of food words, which often resist etymological analysis because they are among the most ancient and heavily borrowed items in any vocabulary.
Latin Roots
The Romance languages show a striking split in their words for cheese. Spanish 'queso,' Portuguese 'queijo,' and Romanian 'caș' descend directly from Latin 'cāseus' — the same word that English borrowed. But French 'fromage,' Italian 'formaggio,' and Catalan 'formatge' come from a different Latin word entirely: 'formaticum' (or 'formaticus'), meaning 'molded thing,' from 'forma' (mold, shape). This word referred to the wooden mold or basket in which cheese was shaped. So the western Romance languages named cheese for what it is made of (curdled milk), while the central Romance languages named it for how it is made (by molding).
The scientific vocabulary of dairy chemistry preserves the Latin root: 'casein' (the primary protein in milk, from which cheese is made) comes directly from Latin 'cāseus.' 'Caseous' (resembling cheese, cheesy) is the formal adjective. 'Casserole' has a more distant connection — it comes from Old French 'casse' (a container), from Late Latin 'cattia,' but the association with baked, melted cheese dishes has reinforced a folk-etymological connection.
In English, 'cheese' has developed extensive metaphorical and idiomatic uses. 'Cheesy' (cheap, tacky, of low quality) is first attested in the mid-19th century, possibly from the idea that cheap cheese has an unpleasant quality. 'The big cheese' (an important person) may come from Urdu or Persian 'chīz' (thing), brought back by British colonials who heard 'the real thing' as 'the real cheese.' 'Say cheese' (a photographer's instruction) dates from the 1940s and exploits the fact that pronouncing the word stretches the mouth into a smile-like shape.
Modern Legacy
Old Irish 'cáise' (cheese) was also borrowed from Latin 'cāseus,' confirming that the Romans introduced both the advanced techniques of cheese-making and the vocabulary for it to the peoples of northwestern Europe. The word's journey from Roman dairy farms to modern English is a monument to the cultural influence of Rome on the Germanic and Celtic worlds — an influence so deep that it reached into the most basic vocabulary of everyday food.