1. an adventure characterized by manliness or extreme masculinity.
2. an epic journey only undertaken by men, or very masculine women.
3. a strenuous task which only individuals naturally equipped with a legitimate, large penis can endure.
"Hey, want to invite Alfred on our manventure through the jungle?"
"Nah, not Alfred. He's too much of a pansy."
A manly adventure, which women cannot partake in, taken on by only the manliest of men (see A.M.M.) which most often includes guns, sharp objects, power tools (though hand-tools are manlier), gas guzzling automobiles, other forms of manly gas powered transportation (see alternate definition of MANventure), pirate maps, AC/DC, sandwiches (made only by women), and an extremely detailed plan that is almost certain to fail. Protocol says an alternate plan is to be promptly made up on the spot no matter how ridiculous, difficult, illegal, or unlikely to be accomplished it may be.
Dude, let's take my home-made submarine across the Pacific Ocean and hunt whales with my buck knife. It'll be a MANventure.
The flagship of American Mans for Manliness (A.M.M.). It is a true man's boat which can be very temperamental and unpredictable at times (see woman). Its home port is located in Bear Creek on the Alabama side of Pickwick lake. It is a true man's badass boat.
Woman: What kind of trash boat is this?
Man: Oh nothing, just the MANventure.
*Woman makes sandwich*
a. Any trip involving two or more men performing manly feats with no planned route or attainable destination
b. Any trip that deviates from the original plans and leaves the women-folk behind
"Dude that was the greatest manventure ever!" (In the context of drinking all night and smoking cigars on the streets of center city Philadelphia)