People who are so obviously gay but aren't telling it. There is a difference between just "in the closet" and "in the GLASS closet".
Those just "in the closet" have speculations circulating around them gay - whether'd about their mannerism or personality, but they're not really pushing their gayness. They will tend to have opposite gender "partners", but many people will conclude that they're just beards/fake.
Those in the "glass closet" are walking and breathing of gayness - e.i., they will be always walking with that one same gendered partner, they adopt children and they might even be reported living with the partner, but they'll never say they're gay or straight.
Queen Latifah, Jodie Foster, Anderson Cooper, Kevin Spacey, Victor Garber (Mr Andrews - the Titanic guy) & Fiona Shaw (Harry Potter's Aunt Petunia) are in the glass closet.
When someone is clearly gay, but is trapped by their own internalized homophobia or other circumstance that prevents them from coming out.
Katrina: “Go fuck yourself Grant”
Grant: “Gaybe I will”
Maddox: “Hey guys let’s keep the love affair on the down low at work”
Katrina: “That’s rich coming from someone living in a glass closet Maddox”
A glasscloset is a person or character who is gay but pretends to be straight, meaning they won't come out of the closet, but it does not matter, because its painfully obvious they are gay.
Or a closet that is see through due to it being made with glass panes.
Ex 1 - Person:
Gabe: Dude, you are such a glass closet.
Andy: Dude I swear I'm not gay!
Ex 2 - Actual closet:
Andy: Hey look at this glass closet you can see through it!
Gabe: Wow, this glass closet it so cool, I want one.
A glasscloset is someone who is gay but claims they are straight, they will not come out of the closet, but it does not matter, because you can see that they are gay.
Or a closet that is see through due to it being made of glass.
Ex 1 - Person:
Gabe: Dude, you are such a glass closet.
Andy: Dude! I swear I'm not gay!
Ex 2 - Actual Closet:
Gabe: You can see the clothes without even opening this closet, that's so cool.
Andy: Yeah because its made of glass idiot.