Definition of "on one's guard" []

• In a watchful state; alert; vigilant. (noun)

Gnu Collaboartive International Dictionary of English: licensed under The Code Project Open License (CPOL)

• "The "on dit" of the mischief-maker is often but a verbal cover for the ill-will he delights to utter, but is not bold enough to acknowledge; and it is very desirable to be on one's guard against those persons who habitually preface their conversation by "they say;" because if they have not originated the maliciousness, they are at least the conveyers of it, and thereby bring it to our ears."