To weigh down; to press heavily upon or burden something physically or metaphorically.
From 'down' plus 'weigh' (from Old English 'wegan', meaning to carry or balance). The compound suggests pressing downward with force or heaviness.
This is an archaic verb form that's mostly vanished from English—language constantly loses words that get replaced by synonyms. 'Weigh down' does the same job and won, so 'downweigh' became unnecessary.
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