Stale beer, nearly sour, is said to be hard. Hard also means severe: as, hard fate, a hard master.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Hans In Kelder * Hard At His A-seNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes