Definition of black-jack:
(n) :
A coalfish (Pollachius virens).
(n) :
A larva of a sawfly of species Athalia rosae (syns. Athalia centifolia, A. spinarum).
(n) :
(uncountable, obsolete, mineralogy, UK) Sphalerite; zincblende.
(n) :
(uncountable, obsolete) Caramel or burnt sugar, used to colour wines, spirits, ground coffee, etc.
(n) :
(obsolete) A large leather vessel for beer, etc.
(n) :
(obsolete) A water oak or barren oak (Quercus nigra).