I had always assumed that press in the sense ‘force someone to become a sailor’ (as in the phrase press gang) was simply a transferred use of the ordinary verb, but a lively book I’ve just stumbled on, The Press-Gang Afloat and Ashore by J. R. Hutchinson (1914), provided a surprising explanation which I (having verified it with more scholarly sources of lexicographical information) hereby pass on to you:
The origin of the term “pressing,” with its cognates “to press” and “pressed,” is not less remarkable than the genesis of the violence it so aptly describes. Originally the man who was required for the king’s service at sea, like his twin brother the soldier, was not “pressed” in the sense in which we now use the term. He was merely subjected to a process called “presting.” To “prest” a man meant to enlist him by means of what was technically known as “prest” money—“prest” being the English equivalent of the obsolete French prest, now prêt, meaning “ready.” In the recruiter’s vocabulary, therefore, “prest” money stood for what is nowadays, in both services, commonly termed the “king’s shilling,” and the man who, either voluntarily or under duress, accepted or received that shilling at the recruiter’s hands, was said to be “prested” or “prest.” In other words, having taken the king’s ready money, he was thenceforth, during the king’s pleasure, “ready” for the king’s service.
By the transfer of the prest shilling from the hand of the recruiter to the pouch of the seaman a subtle contract, as between the latter and his sovereign, was supposed to be set up, than which no more solemn or binding pact could exist save between a man and his Maker. One of the parties to the contract was more often than not, it is true, a strongly dissenting party; but although under the common law of the land this circumstance would have rendered any similar contract null and void, in this amazing transaction between the king and his “prest” subject it was held to be of no vitiating force. From the moment the king’s shilling, by whatever means, found its way into the sailor’s possession, from that moment he was the king’s man, bound in heavy penalties to toe the line of duty, and, should circumstances demand it, to fight the king’s enemies to the death, be that fate either theirs or his.
By some strange irony of circumstance there happened to be in the English language a word—“pressed”—which tallied almost exactly in pronunciation with the old French word prest, so long employed, as we have seen, to differentiate from his fellows the man who, by the devious means we have here described, was made “ready” for the sea service. “Press” means to constrain, to urge with force—definitions precisely connoting the development and manner of violent enlistment. Hence, as the change from covert to overt violence grew in strength, “pressing,” in the mouths of the people at large, came to be synonymous with that most obnoxious, oppressive and fear-inspiring system of recruiting which, in the course of time, took the place of its milder and more humane antecedent, “presting.” The “prest” man disappeared, and in his stead there came upon the scene his later substitute the “pressed” man, “forced,” as Pepys so graphically describes his condition, “against all law to be gone.” An odder coincidence than this gradual substitution of “pressed” for prest, or one more grimly appropriate in its application, it would surely be impossible to discover in the whose history of nomenclature.
Yet another example of the remarkable workings of coincidence, that resented but ubiquitous factor in human affairs.