1.
If you say that you had not reckoned with something, you mean that you had not expected it and so were not prepared for it.
Giles had not reckoned with the strength of Sally’s feelings for him.
= bargain for, bargain on
PHRASAL VERB : with brd-neg , V P n
2.
If you say that there is someone or something to be reckoned with , you mean that they must be dealt with and it will be difficult.
This act was a signal to his victim’s friends that he was someone to be reckoned with...
PHRASE : n PHR