出典:Wiktionary
From the traditional signal from a boxer's support staff of throwing such an item into a boxing ring to indicate that the contestant cannot continue the match and is forfeiting to the opponent.
throw in the towel (三人称単数 現在形 throws in the towel, 現在分詞 throwing in the towel, 過去形 threw in the towel, 過去分詞 thrown in the towel)