“Take the King's shilling”
What it means
To enlist; to accept an obligation by taking payment.
Where it comes from
A recruiting sergeant paid a shilling to any man who agreed to join the army. Accepting the coin was the legal act of enlistment, binding whether or not the recruit had been sober at the time.
How sure are we?
Well attested from the seventeenth century onward. Recruiters were widely accused of dropping shillings into tankards so that a drinker would find he had accidentally enlisted, though how often this actually happened is another matter.