This was the brand name for a self-tanning lotion created in the 1960's, to appeal to a male audience. By extension, any tan that's "man tan" is fake. The expression is commonly used to mean an artificial suntan in both the US and the UK.
2) UK usage:
"The £2.99 man tan on sale"
"No need to hit the beach anymore. It's not usually men who get accused of faking it - but that may be about to change. Preening males can now look bronzed to perfection - with the launch of a cheap fake tan aimed exclusively at them. It gives a gradual tan, so they don't go from Peter Crouch to Peter Andre overnight.
" 'British men are more image-conscious than ever,' said the manager of the supermarket selling it. 'We hope it helps blokes achieve the glow they're after, without the tell-tale streaks.' "
3) USA usage:
" As a native Californian trapped in New York, I’ve watched my former consistently bronze glow fade to a somewhat gray shade of pale. So…what’s a guy to do, you may be thinking? (...) I hit the city streets in search of a sunless man tan. (... I chose) the spa’s 'organic spray' tan, which promises a natural looking tan using a unique combination of mineral salt bronzers, beets, sugarcane and the approved cosmetic tanning agent, DHA 20."
Any tan outline that results from shaving facial hair after getting enough sun show a visible difference in skin color causes the individual to have a "man tan"
You: Yeah I did! I'm going to go shave my face now, so I can show off my man tan.
Friend: Is a man tan is a good thing??
You: Duh! Just wait until you see my man tan, you'll be like: Winning!
Therefore you have a tan line on the middle of your thigh when you wear your bathing suit.
"A freaking man tan!"