This is the 255th most frequent Japanese word.
放題
"放題" means "all you can [do/eat/drink/etc.]."
Here, 放題 combined with the verb 'to use' (使い) describes an unlimited use service.
インターネットが使い放題のプランに申し込みました。
I subscribed to a plan with unlimited internet usage.
In this context, 放題 is used with the verb 'to say' (言いたい) to suggest freedom to speak without limitation.
文句を言いたい放題言ってください。
Feel free to complain as much as you want.
Here, 放題 is used with the verb 'to eat' (食べ) to imply an unlimited consumption option.