Refers generally to
cannabis-based therapy but could also refer to other psychedelics, or more broadly to embracing nonchalance. Pending context, the expression could be an invitation to others to partake in an activity, or a declaration of indifference to
one's carefree decision making. The expression is generally used without punctuation, similar to Marvin Gaye's 'What's Going On', and is also
seen abbreviated to WNM.
"You guys down for some Why Not
Man when we go hiking later?"
"I just bought a pair of $400 cowboy
boots, Why Not
Man"