Entry 1 sense Webster, 1913 Tranquilizer /trăng'-kwəl-īzˌ-ẽr/ · Tran·quil·i·zer · IPA /ˈtɹæ̝ŋkwɪlaɪ̯zɚ/ 01 n. One who, or that which, tranquilizes. 1. One who, or that which, tranquilizes.