-dropone'sguard;becarelessandunconcerned;lackofvigilance;beoffone'sguard;insensitiveandnegligent;neglectful;becaughtoffguard;letone'svigilancebelulled;lowerone'sguardandbecomecareless;slacken[relax]one'svigilance:
Wecan'tgetunwaryjustbecausenothingunusualhascroppedup.不能因为没发生什么问题就麻痹大意起来。