Very bad, poor quality, or unpleasant; also historically meant infested with lice.
From Middle English, derived from Old English 'lūs' (louse). The literal meaning 'infested with lice' was common, and the word evolved metaphorically to mean 'unpleasant' or 'of poor quality' because lice were associated with filth and disease.
The jump from 'infested with lice' to 'really bad' is a pattern we see across languages—body parasites were so universally despised that cultures worldwide used louse-related words as insults. This shows how universal human disgust can shape language in predictable ways.
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