Young, impressionable (scantily dressed) females who hang around trying to get rides on the back of motorcycles.
Also can be the girlfriend of a motorcycle rider. Him in full riding gear, her in a tank top with her thong hanging out the back of her jeans.
Originates from the expression 'having a monkey on your back".
There goes Jim on his Kawi with a curb monkey hanging off of him like a lemur.