Used to describe a situation where the usual rules of attraction no longer apply. Standards are lowered in order to accomadate the less appealing.
Particularly in the workplace, where a 38 year old divorced mother of two on the end of the production line stacking boxes can appear pretty tidy.
H.D:"that bird you pulled last night was a bit shabby rtj."
rtj:"yeah, I was playing by Roussel rules."
To be twice as strong.
In particular, refers to acid that has been 'double dipped', however can also be used to describe a range of potent substances/activities/ people etc
See also fat old boy
Welsh dealer at Glastonbury: "look 'ere boyo, go easy with that- it's double dipped."