Hello and (almost) welcome to the forums
It's a transcription of "je ne sais pas" (I don't know) pronounced a certain way: je ne sais pas => je sais pas => ch'ai pas (very colloquial)
And some people pronounced "pas" like "pô"
That's how most of us (myself included) say it in Québec. I don't know about other French-speaking countries. "Chai" is both used for "Je sais." and "Je ne sais" and "pô" is how "pas" is pronounced here. Bear in mind that it should only be used in speech and/or chat, never written.
Edit: Oh, noticed someone else beat me to it... Should I delete my post?