Often ill or unhealthy; appearing weak or pale; or in a way that suggests disease or weakness.
From Old English 'seoc' meaning 'sick' plus the diminutive or descriptive suffix '-ly.' The word evolved to describe either a chronic condition of being frequently ill or a quality that looks diseased.
Interestingly, 'sickly' can mean both an actual health condition and just looking unhealthy—a plant can be sickly green, a person can have a sickly complexion even if they're not actively ill. It's describing a whole vibe of weakness.
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