Along with this; by means of this document or letter (formal and old-fashioned).
From Old English 'her' (here) + 'with,' combining two common Germanic words. Used extensively in formal documents since the Middle Ages.
You'll mostly see 'herewith' in old legal documents and stuffy formal letters—it's the kind of word that makes official paperwork sound official, even though saying 'attached is' works just fine in modern English.
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