What is the origin of the noun "craic"?

I always assumed the phrase "craic on" was actually "crack on", however my naivety was corrected today when I was informed that the correct spelling is, in fact, "craic".

I have never seen this word before. Where did it come from and what does it actually mean?

It is an Irish word (pronounced 'crack') which means an enjoyable social activity, a good time, with lots of laughter (and usually booze). Someone returning from holiday might say - 'the beach was not very good but the craic was wonderful'. In Scotland and the North of England 'crack' is also used to mean 'conversation'.

Actually, without invalidating WS2's fine answer there is the British English idiomatic phrase (lets) crack on: “Crack on” can mean either “start” or “continue”, depending on the context and progress of the activity

We’ve got a long journey ahead of us. Let’s get cracking. (Let’s go / start now.)
I’ve still got a lot to do so I better crack on

TFD defines it as:

to continue to do something as quickly as possible

Curiously, in Australian English, to crack on to is:
seek to form a sexual relationship with (someone).

Thus depending where or from whom the OP heard the following expressions; crack on, crack on to, or the Irish craic as in

Séamas: "Come into to town, it's great craic!

either one of the three may be correct.