Definition of "on guard" []

• The starting position of a fencing bout; en garde. (adjective)
• Vigilant, alert (adjective)

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• "The editor of Black Mask, Joseph Shaw, was on guard against the use of vulgarisms by his writers."
• "Pickett was left at Chambersburg; Law's brigade was held on guard at New Guilford."
• "Evelington Heights was reached and a Federal squadron on guard there sent ascampering."