An immense amount - possible derived from the amount you can get in a shed - or the description of the amount of crap that covers the road (almost specifically designed to kill motorcyclists) when a lorry sheds its load.
Originally heard as Royal Marines slang.
"What do you fancy doing tonight?"
"I dunno, but Bomber has got a fucking shed load of vodka in his fridge and Sally has just got a copy of the porn movie she starred in"
"I'll get the coke!"