reckon with somebody/something phrasal verb ( see also ↑ reckon )
1 . somebody/something to be reckoned with someone or something that is powerful and must be regarded seriously as a possible opponent, competitor, danger etc:
Barcelona will be a force to be reckoned with this season.
The principal was certainly a woman to be reckoned with.
2 . not reckon with somebody/something to not consider a possible problem when you are making plans:
I had not reckoned with the excitement in the popular press.
3 . have somebody/something to reckon with to have to deal with someone or something powerful:
Any invader would have the military might of NATO to reckon with.