What does cos mean?
'Cos functions primarily as an informal shortening of 'because', used constantly in everyday conversation, especially in Britain, to give a quick reason: "I'm late 'cos the bus broke down." Its casualness makes it a marker of relaxed, spoken register — you would say it aloud or write it in a text message, but not in an essay or business report, where the full word 'because' is expected. The same word, usually spelled simply 'cos', also names a familiar vegetable: the cos lettuce, better known in America as romaine, whose elongated, crunchy head forms the backbone of the classic Caesar salad. This second sense comes from the Greek island of Kos, associated with the lettuce since antiquity. Between its role as the workhorse of casual explanation and its place on the dinner plate, 'cos is a small word that quietly shows up all over daily life.
(informal, chiefly British) Short for 'because'. Used to introduce the reason for something.
"She took an umbrella 'cos the forecast looked grim."
Strictly conversational; avoid in formal or written contexts.
A variety of lettuce with a long, upright head of crisp green leaves; also called romaine.
"Wash and dry two heads of cos for the salad."
Usually written without an apostrophe in this sense; known as romaine in North America.
The plural applies to the lettuce sense; in recipes the singular 'heads of cos' is often preferred.
"The market stall had fresh coses stacked beside the spinach."
That little three-letter word you mutter when explaining yourself has been doing the job of 'because' since at least the eighteenth century.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cos
The informal conjunction 'cos developed in English as a natural spoken contraction of 'because', with usage recorded from at least the eighteenth century and flourishing in colloquial British speech ever since; it ultimately traces back through Middle English 'bi cause' to the Old French 'causa', from Latin 'causa' (reason). The lettuce sense entered English separately, borrowed from the modern Latin botanical name for the plant, which honours the Greek island of Kos where the lettuce was thought to have originated. The two senses are therefore unrelated in origin despite sharing a spelling.
How cos is actually used
The conjunction 'cos is distinctly informal and conversational; it belongs in dialogue and casual prose, not formal writing, where 'because' should be used instead. It is heard more often in British than American speech. The lettuce sense, by contrast, appears mainly in culinary contexts such as salads and recipes.
Easily confused with cos
A cosh is a blunt weapon used as a club, whereas 'cos is an informal word meaning 'because'.
Coz is a common informal spelling of the same word as well as slang for 'cousin'; both spellings mean 'because', but coz can also refer to a cousin.
Cause is a noun or verb referring to a reason or source of something, while 'cos is only an informal spoken equivalent of 'because'.