The word عسعس can mean two things. It can refer to the beginning of the night as
darkness begins, or it can also refer to the last part of the night when darkness begins to fade. The root meaning is of penetrating or moving into something dense. Darkness is dense, so it can mean the initial part of the night when the day moves into darkness, and it can also refer to the last part of the night, as hints of light begin to show up as if penetrating this cover of darkness.51
And by the night as it departs
The word عسعس can mean two things. It can refer to the beginning of the night as
darkness begins, or it can also refer to the last part of the night when darkness begins to fade. The root meaning is of penetrating or moving into something dense. Darkness is dense, so it can mean the initial part of the night when the day moves into darkness, and it can also refer to the last part of the night, as hints of light begin to show up as if penetrating this cover of darkness.51
51 Al-Muʿjam al-Ishtiqāqī al-Muʾaṣṣal li-Alfāẓ al-Qurʾān al-Karīm, vol. 3, p. 1461-1462