ボインボインMeaningsNounNo-adjective1. big breasts; big boobsNoun2. boing-boing (sound)Other readingsぼいんぼいんExamples1従姉妹いとこにはね、もの凄すごいボインボインな子こがいるんだよ？Among my cousins, you know, there's a girl with the most enormous breasts.