An event in which guys on the down low try to act macho to hide their homosexuality. A bunch of guys get together to drink beer or camp or whatever. As soon as one of them calls it "mancation" it's turns into one big homorgy (homo orgy).
straight guy: hey wanna go camping with me and the boyz this weekend.
down low guy: oh, you mean a mancation weekend?
straight guy: NO!!! We're just going camping. No russian cookies, no hot-carl's, no rusty trombones, no searching for inner children. Just camping...