Silly means playful and foolish in a lighthearted way, or showing poor judgment without being truly harmful. When your friend makes silly faces to get you to laugh during lunch, or when you and your siblings start speaking in ridiculous accents just for fun, that's silliness: behavior that's goofy, nonsensical, and meant to entertain rather than make sense.

Silly can also describe something that seems foolish or not well thought out. If someone suggests bringing flip-flops on a winter camping trip, you might call that a silly idea. When a character in a story makes a silly decision, like trading a cow for magic beans, we mean they're acting without good reasoning.

The word isn't usually harsh. Calling something silly is gentler than calling it stupid or ridiculous. When a teacher tells the class to “stop being silly,” she means “settle down and focus,” not “you're terrible people.” There's a playful quality to silliness that keeps it from being an insult.

Young children often go through a “silly phase” where everything makes them giggle: silly songs, silly words, silly walks. Even adults appreciate silliness sometimes. After all, life would be pretty boring if everyone acted seriously and sensibly all the time.