Something off-limits is forbidden or not allowed to be used, entered, or discussed. The construction site near your school might be off-limits to students, meaning you're not permitted to go there. Your older sister's room might be off-limits when she's studying. In a game of tag, certain areas might be declared off-limits so players can't hide there.

Today, we use it for any situation where clear boundaries exist. Parents might declare the fancy living room off-limits before a party, or a teacher might make certain topics off-limits during a debate to keep the discussion focused.

Notice that off-limits implies a rule or authority: someone has decided the boundary exists. It's different from something being merely unavailable. The library isn't off-limits when it's closed for the night; it's just closed. But if your parents say the cookie jar is off-limits before dinner, they've created a rule you're expected to follow.