Green’s Dictionary of Slang

donk n.2

[? both have ‘a kick like a mule’]

1. illicit ‘moonshine’ whisky.

[US] ‘Wet Words in Kansas’ AS IV:5 386: Whiskey is sometimes called donk or mule because of its powerful ‘kick.’.

2. (US drugs) marijuana and PCP.

[US]ONDCP Street Terms 8: Donk — Marijuana/PCP combination.