If people behave rowdily, they are loud, rough, and a bit wild — shouting, shoving, making a mess — in a way that can tip into a fight. It is the adverb from rowdy. You use it about groups more than about one quiet person: fans leaving a match, a late street, a classroom when the teacher has stepped out. It is informal, and it always leans negative. Nobody says “we celebrated rowdily” unless they are half-joking. It does not mean only “happy and loud.” A party can be noisy without being rowdy. The extra bite is the roughness, the feeling that trouble is close.