To defy is defined as to work against or resist something.
(verb)An example of to defy is to break a law by going on land that is posted with a no trespassing sign.
See defy in Webster's New World College Dictionary
transitive verb defied, defying
Origin: ME defien < OFr defier, to distrust, repudiate, defy < LL *disfidare < dis-, from + *fidare, to trust < fidus, faithful: see faith
noun pl. defies
See defy in American Heritage Dictionary 4
transitive verb de·fied, de·fy·ing, de·fies
Origin:
Origin: Middle English defien
Origin: , from Old French desfier
Origin: , from Vulgar Latin *disfīdāre
Origin: : Latin dis-, dis-
Origin: + Latin fīdus, faithful; see bheidh- in Indo-European roots
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