The comedy movie was so hilarious that everyone in the theater laughed riotously.

riotously 是形容词 riotous 的副词形式。riotous 由名词 riot（骚乱、狂欢）加形容词后缀 -ous 构成，本义是"骚乱的、放纵的"，引申为"喧闹的、狂欢的"。riotously 表示"喧闹地、放纵地"，形容笑声很大很疯狂的样子。