"vessel," from late Old English pott and Old French pot "pot, container, mortar" (also in erotic senses), both from a general Low Germanic (Old Frisian pott, Middle Dutch pot) and Romanic word from Vulgar Latin *pottus, of uncertain origin, said by Barnhart and OED to be unconnected to Late Latin potus "drinking cup." Celtic forms are said to be borrowed from English and French.
Slang meaning "large sum of money staked on a bet" is attested from 1823. Pot roast is from 1881; phrase go to pot (16c.) suggests cooking. In phrases, the pot calls the kettle black-arse is from c. 1700; shit or get off the pot is traced by Partridge to Canadian armed forces in World War II.