na-adjective, noun
doing as one pleases; having one's own way; self-indulgent
Describes behavior that is unrestrained, selfish, or without regard for others. Often used in negative contexts to criticize someone's actions.
彼は会議中にし放題で、他の人の意見をまったく聞かなかった。
He did whatever he pleased during the meeting and didn't listen to anyone else's opinions at all.
子供たちが公園でし放題に遊んでいる。
The children are playing freely in the park, doing whatever they want.
Compound of し (continuative form of する 'to do') + 放題 (suffix meaning 'as much as one likes'). The combination lexicalized into a noun/adjectival noun meaning unrestrained behavior.