precautionary adjective 1noun 1forms 3 adjective1 •Of, pertaining to, or serving as a precaution.The port cities were quarantined as a precautionary measure, though no cases of the disease had been found. noun1 •(no longer used) A precaution.Thou seest, Belford, by the above precautionaries, that I forget nothing. — 1748 formsprecautionaries · more precautionary · most precautionary