If someone is boisterous, they are full of energy and they make a lot of noise — laughing, shouting, running about. People use it most about children, and often with a half-smile rather than real anger: the children were boisterous at the party. It is not as harsh as rude or naughty. Adults can be boisterous too, especially in a loud, cheerful group, though about a grown person it can hint that they are a bit much. It describes behaviour in a moment, not a person’s whole character. The older history is messy. It comes from Middle English boistres, an altered form of boistous (“crude, unrefined”), of uncertain origin, likely from Anglo-Norman boystous / bustous (“rough, in a crude state,” also “lame”) and Old French boitous (“limping; having a rough, uneven surface”). The path is not certain, and the meaning has shifted toward noisy energy rather than “rough” or “lame.”