Green’s Dictionary of Slang

1. bad, unpleasant.

1933		E.A. Robertson Ordinary Families 63: Surely even a dank sort of person like Mr. Quest (‘dank’ was Lester’s word for him) who did not care much about his own children, must admire father?
1979		Age (Melbourne) 1 Mar. 2/2: ‘With Frank Victoria will soon stop being dank’.
1989		P. Munro Sl. U.
1999		Guardian Guide 5–12 June 29: A dank world where people wear specs patched up with Elastoplast.
2010		K. Swinson Heist 30: I started realizing this little fucking dank-ass cell might be my fucking reality.
2015		P. Beatty Sellout (2016) 64: Weed that smells bad is good [...] a dank, wispy cloud of smoke that smells like [...] dead fish and seagulls roasting in the hot sun.

2. (also diggity dank) excellent, first-rate [presumably on the bad = good model].

1989		P. Munro Sl. U.
1996		Eble Campus Sl. Sept. 2: dank – very, very good [...] Also diggity-dank.
1997		Da Bomb Summer Supplement 4: Dank (adj.) ideal, exceptional, rare.
2004		Eble Campus Sl. Apr. 2: dank – of high quality, superior. Originally applied to marijuana. This cereal/movie/pair of shoes is dank.
2012		C. Eble (ed.) UNC-CH Campus Sl. 2011.
2016		Dly Pennsylvanian 23 Mar. 🌐 The phrase ‘humorous images’ might not evoke a response, but the phrase ‘dank memes’ gets a hearty chuckle, even though they mean the same thing.