Definition of craic in English:

1970s: Irish, from English crack. The English word apparently entered Irish English from Scots in the mid 20th century and subsequently assumed an Irish Gaelic form.

• crack from Old English:

  In Old English crack meant ‘make a sudden sharp or explosive noise’. The drug known as crack, or crack cocaine, is a hard crystalline form of cocaine broken into small pieces and smoked. It gets its name from the ‘cracking’ noises the crystals make as they are heated. The ‘crack’ or lively socializing in a pub is an Irish use, first recorded in the 1920s and sometimes written craic, that comes from the Scottish sense ‘chat, conversation’. You can talk about a time very early in the morning as the crack of dawn. The expression is first recorded in the late 19th century, in the form crack of day. The crack here is the crack of a whip, with an additional echo perhaps of break of day and daybreak, and the notion of the sky cracking or breaking open to reveal a sliver of light. The crack of doom is a peal of thunder which, according to the Book of Revelation, will announce the Day of Judgement. See also paper, pop