reckon without somebody/something phrasal verb British English (
see also reckon)
if you are reckoning without something, you do not expect it and are not prepared for it:
They doubted that Fiona could finish the course, but they reckoned without her determination. [TahlilGaran] Dictionary of Contemporary English ▲
reckon without v. To fail to consider as one of the things which might change a situation; not think about.
The committee for the class picnic party made careful plans for a beach party but they reckoned without a sudden change in the weather. [TahlilGaran] English Idioms Dictionary ▲