... is what is known in law as an 'earnest payment' – basically, a deposit taken to demonstrate that the applicant is serious about wanting to complete a transaction. The king's shilling was paid by the army (or navy) to demonstrate that the recruit had been accepted into the king's service and would be paid accordingly in the future.
Thus, the legal obligation created by the king's shilling lay with the armed service, not with the recruit. The recruit was entitled to return the shilling, and only became subject to martial law upon formal attestation before a Justice of the Peace.
Press gangs had the power to compel British seamen to join the Royal Navy. Many merchant sailors were impressed, as well as some sailors from other nations, but it was rare for non–seamen to be impressed. Impressment could only legally be applied to "eligible men of seafaring habits between the ages of 18 and 55 years".
The idea that unsuspecting targets were pressed after a shilling was slipped into their drink, and that this was the origin of the glass–bottomed tankard, is probably a myth. The Navy could press by force, so there was no need for deception. Also, as explained above, the legal obligation created by the king's shilling lay with the armed service and not with the recruit.
Impressment in Britain ended, in practice, at the end of the Napoleonic Wars (1814). Legal changes were made in 1835 and 1853, but by this time the Navy had enough volunteers to meet its manpower needs. Conscription, to all military services including the Navy, was introduced during World War I.
The practice of recruits being given 'the King's shilling' officially stopped in 1879, but the term is still used informally.
The above has been extracted (and edited) from various pages on Wikipedia.
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