To soak or saturate something completely with liquid, usually water; to make something extremely wet.
From Old English 'drencan' meaning 'to drink' or 'to force to drink,' related to 'drink.' The meaning evolved from 'forcing liquid down' to the more general sense of soaking something thoroughly.
In veterinary medicine, 'drenching' a sheep means pouring medicinal liquid directly down its throat—it's still using the original Old English meaning of 'forcing to drink' that survives nowhere else in modern English!
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