When someone calls a person a sheep, they mean that person follows along with what everyone else does without thinking for themselves. If all your classmates suddenly decide recess soccer is boring and switch to basketball, and you join them even though you still love soccer, someone might say you're acting like a sheep.

The insult comes from how real sheep behave: they tend to move together as a group, or flock, following wherever other sheep go. Farmers and shepherds know that if you get one sheep moving in a direction, the rest often follow along behind it.

But calling someone a sheep isn't really about the actual animal. It's about choosing to go along with the crowd instead of making your own decisions. When your friend refuses to pretend to like a popular TV show she finds boring, she's showing she's not a sheep. When someone stands up for a person being teased even though others are joining in, that shows they can think for themselves.