Jeroboam of Rum or Claret
(A). Eight bottles; but of whisky three pints. Probably a perversion of “joram.” (See Tappit-Hen and Rehoboam.)
“Some `jeroboams' of very old rum went at each; several `tappit-hens, of rum fetched and some `magnums,' 17s. each.” —Truth, 31st March, 1887.
A magnum = 2 quart bottles; a tappithen = 2 magnums; a jeroboam = 2 tappit-hens; and a rehoboam = 2 jeroboams or 16 quart bottles.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894