Wanking on a plane, usually in the plane lavatory. Like the Mile High Club but far easier to attain and much more shameful.
Example 1
Guy1:Johnny's been in the bathroom awhile, I bet you he's joining the Mile High Club.

Guy 2: Pfft maybe as a half member!
Johnny: Guess who's now a Mile High Club Half Member!

Guy 1: Called it!

Example 2
Drunk guy:Never have I ever joined the Mile High Club!

*Johnny guy drinks*

Johnny:I'm a Mile High Club half member!
Drunk guy: Ew man gross!
Johnny:Hey dude, it was a long flight and I was bored.
by mrperson123 January 30, 2018
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