Salam, You can say : "لاَ تَــكُـنْ غَــبِـيّــاً / أَحْـمَــقـاً" if you want to hear the correct pronunciation just click on the link below, then click on the "headphone" icon : http://goo.gl/VBXymv

لا تكُن غَبِيًا la takn ghabiyan