If you are told something is about to happen ( forewarned) you
have time to be prepared ( forearmed).
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Well, forewarned is forearmed...
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If you know about something beforehand, you can prepare for
it.
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If you are warned ahead of time (forewarned) of trouble, you can
prepare yourself for it. Forearmed means "armed ahead of time",
having your weapon ready, or figuratively being prepared.
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The motto of Royal Observer Corps is 'Forewarned is
Forearmed'.