Cos
Cos has 2 meanings
Because
Cos is a common abbreviation online for "because" cos it's shorter to type (you get it?). It is one of many abbreviations for because, along with cuz, bc, cz, bcz, and bcos.
You will most likely see cos in texts, emails, and online in messages and social media. For example, your brother might text you, "Are you going to mom's and dad's 2nite cos I don't want to be stuck with them." Or, your socially awkward friend may post on Twitter, "I like gas stations cos I enjoy the smell of gas. Is that weird?"
Example
He better not ask me out cos imma say no
Oh c'mon! He's cute
Uhh, not my type
How well do you know your Fantasy Football slang?
COS is a warning meant to alert the recipient that a child is watching over their shoulder, so don't send anything inappropriate. It is similar to the POS acronym sometimes used by kids to warn friends that their parent is watching their device.
Parents may send COS when their kid is sitting in their lap and can see their phone when texting. Or, a sitter may send it when messaging a friend online, and a child is near them.
However, since COS is rather obscure, it may confuse the recipient more than actually be helpful. Therefore, unless the person you are sending it to is slang-savvy, you should probably just spell it out instead of using the acronym.