to repeat something again and again in order to make someone understand it
It's an important principle and it has to be hammered in.
phrasehammer in hammer sth in • hammer sth into sb to repeat something again and again so that people completely understand it or accept that it is true or right: Principles of right and wrong have been hammered into us since childhood.Just telling them once won't work. You've got to hammer it in.■ SIMILAR TO: drum into, drill into