Green’s Dictionary of Slang

cougar milk n.

also cougar juice
[note the 1930s–50s US skiers’ drink ‘Cougar Milk’, a blend of condensed milk, rum, nutmeg and boiling water; orig. known as ‘moose milk’, the recipe was printed on the back labels of then popular Coruba Rum]

(US) rough, illicit whisky, esp. that sold during the Prohibition (1920s).

[US](con. WW1) E.C. Parsons Great Adventure 90: [S]quabbles among the boys resulting from strained nerves or too much cougar milk.
[US]McCulloch Woods Words 39: Cougar ‘juice’ [...] a. measure of strength, particularly the kind of whiskey which will rot the bottle. [...] Cougar milk – Any kind of stout home brew or other liquor.
L. Sorden Lumberjack Lingo 28: Cougar milk – Prohibition-era woods liquor.