Mind your Eye
Be careful or vigilant; keep a sharp look out; keep your eyes open to guard against mischief. School-boy wit, Mens tuus ego.
“`Perhaps it may be so' (says I); `but mind your eye, and take care you don't put your foot in it.'” —Haliburton.
“`You must mind your eye, George; a good many tents are robbed every week.'” —C. Reade,
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894