Fast and loose was the name of an old fairground game in which a punter was challenged to pin an intricately folded belt, garter or other piece of material to a surface.
The person running the game would inevitably show that the item had not been securely fastened or made fast and so the punter would lose their money. The phrase came to be used to indicate inconstancy.
1996 - Time Out - The big MGM production typically plays fast and loose with the facts, so it's as much an action spectacular as a genuine historical chronicle.
RELATED IDIOMS :
fast and furious
lively and exciting
2000 - Independent - We understand that the bidding was fast and furious right up to the last minute.
in the fast lane
where life is exciting or highly pressured
pull a fast one
try to gain an unfair advantage by rapid action of some sort – informal
This phrase was originally early century US slang and is also found as put over a fast one.
1993 - What Mortgage - We also know what prices should be and will pull up any builder trying to pull a fast one.