1
general::
Prepared for withstanding aggression or attack, as in The debate team's plan was to keep their opponents on the defensive, or This teacher put students on the defensive about their mistakes. [c. 1600]
American Heritage Idioms
2
general::
Phrase(s): on the defensive
weary and ready to defend oneself. • John goes on the defensive when his athletic ability is questioned. • The child was on the defensive when questioned about cheating.
McGrawhill's American Idioms And Phrasal Verbs