Green’s Dictionary of Slang

1. anything stupefying; thus (W.I.) locus-ale, an intoxicating drink made from the scum of the sugar cane.

1693

T. Blount Natural Hist. 146: The first of which [viz. scum of sugar-cane] that ariseth is little worth; but afterwards, what is scumm’d off, they make a very good drink of, called Locus-Ale, much used by the Servants in Jamaica; or else they convey it into a Copper- Still [...] to be distill'd .

2. (also locuss, locust) a drink, drugged with either laudanum or snuff.

1859		Hotten Dict. of Modern Sl. etc. 60: locuss generally consists of snuff and beer.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor III 387/2: If I had sung, some of the convicts would have given me some lush with a locust in it (laudanum hocussing), and when I was asleep would have given me a crack on the head.
1949		Monteleone Criminal Sl. (rev. edn) 147: locus Narcotics [...] locust [...] knock out drops.
1959		J.E. Schmidt Narcotics Lingo and Lore 103: Locus – 1. An opiate. 2. Any narcotizing preparation. Locust – same as Locus.