Fine, good, valuable.

n. Generically, fiery liquors that produce madness in total abstainers.

like its opposite, queer , was formerly a much-used prefix, signifying fine, good, gallant, or valuable; perhaps in some way connected with Rome. Nowadays it means indifferent, bad, or questionable, and we often hear even persons in polite society use such a phrase as, “What a rum  fellow he is, to be sure,” in speaking of a man of singular habits or appearance. The term, from its frequent use, long since claimed a place in our dictionaries; but, with the exception of Johnson, who says rum , a cant word for a clergyman(!), no lexicographer has deigned to notice it.

“Thus rumly  floor'd, the kind Acestes ran,And pitying, raised from earth the game old man.”
Virgil's Æneid, book v., Translation by Thomas Moore.