Noun, slang.

1. A person, most commonly a millennial, who is a gamer, whose great life experiences are almost exclusively indoors, and often defined primarily in terms of virtual explorations, such as those in the video games Super Smash Bros. and Monster Hunter. A "great indoorsman" could simultaneously be one of two Smash Bros.

2). A person, most commonly of the nerd varietal, who could go outside, but chooses to spend 99.99% of his or her time inside, often stewing in a pile of his or her own filthy and odorous laundry.
Friend: Are Logan and Jeremy going to the bonfire tonight?
Other friend: No, those great indoorsman are going to hang out in their room to game like fiends until their thumbs bleed.
by Definerator November 19, 2016
A person who stays inside Harry's Chocolate Shop (A bar at Purdue University) from open (11:30am) to close (3:00am). Harry's Chocolate Shop is the home of the saying "Go Ugly Early"... a saying used to get into the former speakeasy in the basement.
The aeros at purdue organized an event and many became considered the great indoorsman.
by peacedout December 6, 2010