Cub
A younger or smaller bear: a hairy, often stocky gay man on a lighter frame, who usually likes the company of bigger, older bears.

A cub is a younger or smaller bear: hairy, often stocky, with the same beard-and-belly charm as the big guys, just on a lighter frame and with fewer gray hairs. Cubs are adult men, usually in their twenties or thirties, and the word is about the place they hold in the bear family, not about being boyish.
There is also an attitude that goes with it. A cub is often the one who likes older bears, gets bear-hugged at the bar and is quite happy to be the little spoon. Not always, though: plenty of cubs date other cubs, and some simply call themselves cubs because "bear" does not fit yet.
Where it comes from
The word grew out of bear culture. Bear for "a hairy, beefy homosexual male" is in print in the US by 1979, and over the next decades it became a whole community built around an unpolished, everyday masculinity. Once there was a bear, there had to be a cub. Green's Dictionary of Slang has "a daddy bear and his little bear cub" in a 1991 novel, and cub on its own as "a younger, and perhaps smaller man, who enjoys the company of bears" in a 2003 gay glossary.
Cub, otter or twink
The bear family is a zoo, and the animals overlap. A cub is hairy and soft or solid; an otter is hairy but slim; a twink is young, slim and smooth, which puts him at the far end of the spectrum. A cub who keeps going to the gym may end up a muscle bear, and a cub who keeps aging will one day wake up a bear, then a daddy, then a polar bear.
How it is used today
On the apps and at bear events, cub is a self-label as much as a description. Men put it in their profiles to say both what they are and what they want: a cub looking for a bear is one of the classic pairings of the scene. The man who is into cubs is sometimes called a cub chaser.
Related terms
Sources 3
- Green, Jonathon. "bear, n." sense 6c and compound bear cub. Green's Dictionary of Slang, online edition. https://greensdictofslang.com/entry/uxduejy
- Hennen, Peter. "Bear Bodies, Bear Masculinity: Recuperation, Resistance, or Retreat?" Gender & Society 19, no. 1 (2005): 25-43. https://doi.org/10.1177/0891243204272153
- Green, Jonathon. "cub, n.2" Green's Dictionary of Slang, online edition. https://greensdictofslang.com/entry/v5zdxhi