To make someone feel ill or disgusted; to cause physical or emotional revulsion.
From Old English 'seoc' (sick) plus the verb-forming suffix '-en.' The verb form means to make or become sick, either physically or emotionally. The root goes back to Proto-Germanic languages, making it very ancient.
What's interesting about 'sickens' is that it works for both physical nausea and emotional disgust—'that behavior sickens me' uses the same word as 'the smell sickens me,' showing how our bodies and emotions are connected in language! This blending shows ancient understanding of how emotions affect us physically.
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