If youre English, crack is an old English word meaning "lively conversation in good company"; it has been borrowed by the Irish, and respelt as"craic" to give it a history as an Irish word for tourist brochure purposes. If youre Irish, "craic" is an old Irish word, meaning "lively conversation in good company, accompanied by bodhrans": it has been borrowed by the English, and respelt as "crack" to make it look like an old English word. In a recent thread on this subject,contributors supported the English origin, but I dont think any serious linguistic scholars have yet arrived to settle the matter.