early 13c., likur “any matter in a liquid state,” from O.Fr. licour, from L. liquorem (nom. liquor) “liquid, liquidity,” from liquere “be fluid.” Sense of “fermented or distilled drink” (especially wine) first recorded c.1300. To liquor up “get drunk” is from 1845.
It is semi-ironic that this term is being applied to one of the few TTB products that is not intended to be consumed in a “liquid state.” From way back in 1892, here is a court struggling with the term, and trying to find the distinction between beer and liquor.
In a further irony, the term is probably used more commonly, these days, on malt beverages (such as Colt 45) compared to distilled spirits. “Malt liquor” goes back to at least 1937, and Alvin Gluek secured a patent on it in 1948.