This is probably one of those things I once knew but then forgot: porter “A dark-brown or black bitter beer, brewed from malt partly charred or browned by drying at a high temperature” is (according to the OED) “App[arently] short for porter’s ale … The beer was app. orig. either made for or chiefly drunk by porters and the lower class of labourers: compare the early quots. It probably arose as a popular descriptive term.” The first cite for porter’s ale is from Pope, A Further Account of the most Deplorable Condition of Mr. Edmund Curll (1716): “Nurs’d upon Grey Peas, Bullocks Liver, and Porter’s Ale”; that for porter just five years later, from Nicholas Amherst’s Terræ filius: or the secret history of the university of Oxford 1721–22 (27-30 May 1721): “We had rather dine at a Cook’s Shop upon Beef, Cabbage and Porter, than tug at an Oar, or rot in a dark, stinking Dungeon.”
I decided to see if I could find Pope’s A Further Account online, and sure enough, Google Books has it, so I am able to provide a fuller context for the citation. Pope is mocking the unfortunate Mr. Curll, to whom he has administered an emetic in revenge for behavior that displeased him, and he imagines Curll as saying:
Now G―d damn all folios, quartos, octavos, and duodecimos! ungrateful varlets that you are, who have so long taken up my house without paying for your lodging! Are you not the beggarly brood of fumbling journeymen! born in garrets among lice and cobwebs, nursed up on grey peas, bullocks liver, and porters ale?――Was not the first light you saw, the farthing candle I paid for? Did you not come before your time into dirty sheets of brown paper?――And have not I clothed you in double royal, lodged you handsomely on decent shelves, laced your backs with gold, equipped you with splendid titles, and sent you into the world with the names of persons of quality? Must I be always plagued with you? Why flutter ye your leaves and flap your covers at me? Damn ye all, ye wolves in sheep’s clothing; rags ye were, and to rags ye shall return.
Pope had an impressive facility with invective, and you didn’t want to get on his bad side if you could help it.