On the internet, when talking about memes, a "dank meme" means old and out-played. According to Wiktionary, "dank" means "to moisten, dampen; used of mist, dew etc.". This doesn't sound like the definition of "dank" on the context of memes at all.
I would like to know why the word "dank" was chosen.
Best Answer
I would go out on a limb and say that the metaphor underlying dank here is of something old and relegated to a damp basement, edit: or something wet and smelly like vegetable matter in a wet place.
Dank weed is a 150-year-old collocation referring to such plant matter, and that's how it got applied to (top quality) marijuana, whose skunky smell and funky appearance resembled a weed from a moist place and beginning to decompose.
So, according to knowyourmeme.com, "dank" is a term of approbation (like "cool") but is being applied ironically to mean just the opposite,
not cool
.