A steg is well known Glaswegian slang for a person who is particularly well endowed. Usually arises after 5 a sides football, during banter in the showers. The ongoing assumption is that black football players tend to be "stegs". Origin is unknown, perhaps an alcohol-fueled pronunciation of stag.
Alright, fuckin steg ya mad cunt?! Christ you were wielding that shit like a chainsaw last night, almost took my eye out!