also: ひゃくにじっパーセント
adverb
120 percent; absolutely; definitely
Colloquial intensifier meaning 'completely' or 'without a doubt', often used for emphasis in casual speech.
その計画は１２０％成功するよ。
That plan will definitely succeed.
彼の言うことは１２０％信じていい。
You can absolutely believe what he says.
From the literal meaning '120 percent', used hyperbolically to express exceeding full certainty or effort.