Falernum n.

Carribean liqueur made from rum, lime juice and sugar, 1842. Etymology uncertain. An unlikely story is that it is a corruption of the phrase "You have to learn them." OED has no entry for this word but mentions a "chiefly poetical" type of ancient wine called Falerne or Falernum (in this sense, mid-15c.), documented by Pliny, which perhaps could have suggested the name; an 1806 account of a visit to a Guiana sugar plantation mentions the serving of "Falernum wine."

Falernum or drink of contradictions -- Falernum is made in the same manner and contains the same ingredients, in the same proportions, as shrub; with the addition of four gallons of water to every three gallons of rum. Proportions are thus -- one, two, three, and four.

 Of lime juice one, to make it sour;

 Of sugar two, to make it sweet;

 Three of rum, to make it strong;

 Of water four, to make it weak.

[Robert Duff, "British Guiana," 1866.]