man cave
n. An area of a house, such as a basement, workshop, or garage, where a man can be alone with his power tools and projects.
Examples
2004
The basement or garage has become such a special place for special man-projects that DIY is even devoting special programming to it: "My Ultimate Workshop," a one-hour special scheduled for May, looks at tricked-out garages and basements where guys hone their crafts, be it woodworking, car restoration, wine collecting or model-train building.

So how did the man cave make such a transformation? The experts said there are several factors at play: more disposable income, better gadgets on the market for trading up, keeping up with the Joneses and the post-9/11 cocooning factor.
—“Cave dwelling,” Chicago Tribune, March 23, 2004
2004
Maytag's $ 607 Skybox, Fred Lowery says, "is not a refrigerator. It's truly a vending machine."

It's billed as the first personal beverage vendor for home use. More specifically, says Lowery, who directs Maytag's strategic initiatives group, it's meant for your "man cave."

He says company research indicates "every guy would like to carve out his own little place in his home. Internally, we call it the man cave. And lots of guys, at some point, would like a vending machine in their man cave."
—Michael Hiestand, “Skybox? It's a guy thing,” USA Today, January 29, 2004
1992 (earliest)
Evidently, basements are equipped with various power tools and an assortment of sharp bits of hardware that take men hours to examine and the knowledge of which they feel women should not share in. This ensures that on those Saturday afternoons during which there is no televised football, he can slip the surly bonds of day-to-day chatter and chores and disappear into the basement with the simple statement "I have to do a few things around the house."

He can stay submerged for hours, emerging with a satisfied look on his face (which now sports new growth), claiming success in that he "finally got the dang thing working," when really, all he did was plug it in. But with his cave of solitude secured against wife intrusion by cold floors, musty smells and a few strategic cobwebs, he will stay down there for hours nestled in very manly magazines and open boxes of tools.

Let's call the basement, man cave.
—Joanne Lovering, “Hers and hers closet is far more precise,” The Toronto Star, March 21, 1992