The word "craic " is an irish word that is used in the north of england and also scotland for the banter and fun and enjoyment people have with eachother. Adding "top" to this and changing craic to a more simpler crack gives it a more ironic use " topcrack".
Mick:" Pure Topcrack"
Jimmy:" Alright lads whats happening, whats going on, you looking forward to today?"
Woody:" Fuck off jimmy you fucking Topcrack"