A Glaswegian colloquial term of endearment for Santa Claus/ St. Nicholas/ Father Christmas
A jolly, fat old bastard, who breaks into peoples houses every 24th December, steals all their cookies and milk, then leaves toys and gifts for young, spoiled, undeserving little fuckers more commonly known as "children".
Glesga slang : "Auld Slanty Baws will be emptyin his sack at ma gaff the night."
Translated into proper English "Jolly Old St. Nick will be visiting my house this evening to leave some festive gifts!!"