sick·ly / ˈsiklē/ • adj. (-li·er, -li·est) 1. often ill; in poor health: she was a thin, sickly child. ∎ (of a person's complexion or expression) indicative of poor health: his usual sickly pallor. ∎ poetic/lit. (of a place, climate, or time) causing or characterized by unhealthiness: a deep sickly vaporous swamp.2. (of a flavor, smell, color, or light) so unpleasant as to induce discomfort or nausea: the walls were painted a sickly green she liked her coffee sweet and sickly. ∎ excessively sentimental or mawkish: a sickly fable of delicate young lovers.DERIVATIVES: sick·li·ness n.