This is a synonym for a large, double ended dildo. It is typically used when one of your young children discovers your dildo stash and is so young and innocent that they think it is a play toy.
As inquisitive young children they will, naturally, ask you what the toy is and you are forced to respond with the answer that it is a Wobbly Sausage as they are too young for a sex-ed lesson.
Child: mummy what is thisnew toyyou got me? *shaking the dildo with joy*
Mum: that's not a toy it is a Wobbly Sausage
Eric: Dude, did you take Tara home from the Molle Frijole last night?!?
Ferg: Fuck Yeah, Snake! I was all set to plow when the ole Wobbly Goblin made an appearance. I Ended up pushin rope for an hour before she passed out... Then another half hour after that..