Founded long before the Napoleonic wars, the Impress service came into high profile during the wars with Revolutionary France. The word impress was derived from the old French word 'prest', modern 'prêt' or loan/advance, in other words, each man 'impressed' received the loan of a 'shilling' (that is he paid the 'King's shilling' to enlist) and became a '(im)prest man'. The service, also known as the Press Gang, was present in every major port in the kingdom. The service's offices were called 'Rendezvous' with a Regulating Officer in charge, and he hired local hard men as 'gangers'. These thugs would thus roam the countryside attempting to 'encourage' men aged between 18 and 55 to join the navy. No-one was safe from the gang, and often the only escape route when captured was to bribe the gang or to join it. A preferred target for the pressgang was the merchant navy, so it was not infrequent to find special hiding places on merchant vessels. Also, the return of prisoners of war from France was also seen as the perfect moment to impress crewmen, such that very often the returning POWs were turned round and pressganged even before they set foot once more on home soil. The captains of merchant vessels frequently took pity on those they were repatriating and tried to let them land in places far from the ports and the pressgangs.
Hence the expression to be 'pressganged' into doing something, meaning to be forced into doing it.