A buck is an adult male deer, antelope, reindeer, or rabbit. You can usually tell a buck by its large horns. If it’s a bunny, you just have to ask. A buck is also slang for an American dollar.
Full-grown male kangaroos are bucks, too. When a male animal is a buck, the female is a doe. A buck is a male deer, and so a doe is a female deer. When it's used as a verb, buck means to move in an abrupt, jerky way — when a horse bucks, it jumps and kicks its back legs out behind it. If someone bucks the system, they’re pushing against it. Colloquially, "to buck up” means to cheer up.