Play fast and loose
Be inconstant and unreliable.
This derives from an old deception or cheating game in which something that appears stuck (fast) easily becomes loose. It is nicely defined in James Halliwell's A dictionary of archaic and provincial words, obsolete phrases, proverbs and ancient customs, from the fourteenth century, 1847:
"Fast-and-loose, a cheating game played with a stick and a belt or string, so arranged that a spectator would think he could make the latter fast by placing a stick through its intricate folds, whereas the operator could detach it at once."
A 14th century text from George Whetstone The right excellent historye of Promos and Cassandra, 1578, which is one that Halliwell may well have been referring to, says:
"At fast or loose, with my Giptian, I meane to haue a cast."
The first known figurative use of the term is from Tottel's Miscellany, 1557:
"Of a new maried studient that plaied fast or loose."
Shakespeare later used it in King John, 1595:
Heaven knows, they were besmear'd and over-stain'd
With slaughter's pencil, where revenge did paint
The fearful difference of incensed kings:
And shall these hands, so lately purged of blood,
So newly join'd in love, so strong in both,
Unyoke this seizure and this kind regreet?
Play fast and loose with faith? so jest with heaven,
See other phrases and sayings from Shakespeare.