In "Tower of the Elephant", Robert E. Howard uses the word "drinking-jack" three times apparently meaning mug or something like that, judging by the context:
Torchlight licked luridly from broken windows and wide-thrown doors, and out of those doors, stale smells of wine and rank sweaty bodies, clamor of drinking-jacks and fists hammered on rough tables, snatches of obscene songs, rushed like a blow in the face.
'Then give ear and learn wisdom, fellow,' said he, pointing his drinking-jack at the discomfited youth.
Ale splashed over the jack's lip, and the Kothian roared in fury, dragging at his sword.
However I can find no meaning which fits in any dictionary or anywhere else. I know that he was from Texas and lived from 1906 to 1936, so is this a Texan colloquial term or something? Does it even mean mug or am I misunderstanding it?