Bad port wine. A sailor's name for any bad liquor. In North America, “Black-strap” is a mixture of rum and molasses, sometimes vinegar is added.
“The seething blackstrap was pronounced ready for use.”—Pinkerton: Molly Magaires, chap. xvii. p. 174.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894