underarm /ˈʌndɚˌɑɹm/noun 2adjective 1verb 1forms 3 noun2 •[before a noun] The armpit. •Old-fashioned and now outlawed style of bowling in which the arm is not swung over the shoulder. adjective1 •For use under the arm.A 1997 study found that about 104 million American women were using underarm deodorant products. — 2001 verb1 •To supply with insufficient weaponry. formsunderarmed · underarming · underarms