Immediately; right away without delay.
An Old English compound combining 'forth' (forward) and 'with' (along), literally meaning 'forward with.' By the Middle Ages it had evolved to mean 'immediately' because moving 'forth with' something meant doing it without hesitation.
This word sounds like it belongs in a medieval castle, and you'd be right—kings and queens used 'forthwith' in royal decrees to emphasize urgent commands that had to be obeyed instantly, making it one of the original power-words.
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