According to Arabic lexicons, the word means vigilance or a cautious fear, but it only occurs twice in the Qur’an, and in both instances it implies terror.
Surah ii. 18: “They put their fingers in their ears at the thunder-clap for fear of death.” (Hazara ‘l-Maut.) Idem. 244: “Dost thou not look at those who left their homes by thousands for fear of death.”
Based on Hughes, Dictionary of Islam