Term adopted from Japanese into the English used chiefly by Japan-dwelling gaijin, meaning to overcome, by will, a deficiency in genkiness, i.e. the degree of being genki, where genki has the Japanese meaning of possessing high degree of health, vitality and/or energy.
Use of this term has nothing to do with being Wapanese if used by an expat living in Japan or when conversing with current or former inhabitants of Japan.
Not to be confused with any of the definitions provided of genk (or any other English language terms adopted from non-Japanese language origin).
Whaddya mean you're not drinking tonight? Genk the fuck up, man! (in this sense, compare harden up, harden the fuck up)
Ahh, you're sick, huh? Hope you genk up soon!