cos

cos — conjunction

1. used in informal speech or writing instead of 'because' to introduce the reason

1.conjunctionB1
Definition

used in informal speech or writing instead of 'because' to introduce the reason why something happens or why someone does something

Example

Owen stayed home from school cos he had a high fever and a sore throat.

cos + clause stating the reason

Kemi was smiling all afternoon cos she got the scholarship she applied for.

Synonyms
  • because

    standard, neutral register; suitable for all contexts including formal writing

  • since

    more formal and slightly more distant in tone; often used in writing

  • as

    formal, common in written English; often begins the sentence

Grammar Patterns

cos + clause stating the reason

Usage Note

Common in casual texting, notes, and everyday conversation. Avoid 'cos' in formal essays, business writing, or academic papers — use 'because' or 'since' instead.

Common Mistakes

Cos it is raining, I brought an umbrella.
I brought an umbrella cos it was raining.
'Cos' usually starts the second clause (after the result), not the first. But placing it first is also heard informally.
He left early cos of the traffic.
He left early cos the traffic was bad.
'Cos' is a conjunction that connects a full clause; use 'because of' before a noun phrase.

cos — noun

cos — abbreviation

Word family

Synonym discussion

Sense: conjunction/1

'Cos' is the most informal of the four ways to give a reason, common in casual speech, text messages, and personal notes. 'Because' is the neutral all-purpose choice, suitable for both conversation and most writing. 'Since' is slightly more formal and often used when the reason is already known to the reader — it also carries a time meaning, which can cause ambiguity. 'As' is the most formal of the set and typically begins the sentence, creating a more literary or academic tone. For example: 'I left early cos I was tired' (spoken to a friend); 'I left early because I was tired' (neutral); 'Since I was tired, I left early' (formal, reason known); 'As I was tired, I decided to leave early' (formal, literary). Choose 'cos' only in informal settings where you would also use contractions like 'I'll' or 'don't'.

because ↗sinceas ↗

Etymology

LatinEnglish

The conjunction 'cos' first appeared in written English in the early 1800s as a shortened form of 'because', itself formed from the Middle English phrase 'by cause'. It was originally spelled 'cause' or 'caus' before settling into the modern 'cos' form in informal writing. The mathematical abbreviation 'cos' comes from the Latin 'cosinus', a shortened form of 'complementi sinus' ('sine of the complement'), coined by the English mathematician Edmund Gunter in the early 1600s. The two senses — the causal conjunction and the trigonometric abbreviation — developed independently and are unrelated in origin.