What does " 'Cos" mean please? Is it a word?
This's a song in our English book, and I notice the " 'Cos" there.
the Song
Last day of school
1=C 4/4
It's the last day of school and I know what I'm go-ing to do this sum-mer. 'Cos I'm on ho-li-day, I'm going to stay in bed all day. I'm going to get up late this sum-mer. 'Cos I'm going on ho-li-day, I'm going to lie on the beach and swim in the sea. Come and see me there, that's where I'm going to be. But may-be I should do some work. May-be I should learn some new Eng-lish words. May-be I should. May-be I should. But it's the last day of school and I'm go-ing to have a rest this sum-mer. What a-bout you?

Question

Answer

It means "because."

It's not a word. It is a phonetic spelling of the way some people say "because" when they are being casual, or speaking quickly.

First, it is common to shorten "because" to "'cause." Second, "'cause" may be spelled "'cos" or "'cuz" because that is sometimes the way people say it.

The person in the song is speaking in the first person, in a casual and informal way.

In books, you will often see phonetic spellings of this kind in dialog, within quotation marks, when the author is trying to convey the actual sounds of nonstandard, colloquial English.

Answer

It is short for "because"

Answer

I agree. 'Cos' is like all of those other non-existent words, like 'wanna' and 'gonna', that appear in songs and subtitles. We native speakers don't write them - we use normal standard English, even in informal contexts. When non-natives write them it just looks like they haven't learnt English properly.

Answer

It's good to know that 'cos means because, but always, always write "because". I see learners using 'cos because they think it's natural but the truth is, it makes the writer seem very poor at English. Just follow standard English and you'll have no problems - that's exactly what native speakers do.