Much like the movie, The Labyrinth, a "bowie knuckle" is an apple-sized lump in the male anatomy protruding in the groinal region. Particularly emphasised by tight spandex pants, or jeans. See also pud.
Girl: That guy had the hugest Bowie Knuckle I've ever seen!
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In forum terms, OP refers to the Original Poster. The person who began the thread.
ForumGeek911: So yeh, the OP said you were dumb.
OP: That's right, I did!
ForumGeek911: OMG, He just necroposted.
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