What is the origin of the idiom "reckon without somebody or something" and when to use it?

The origin of the idiom "reckon without somebody or something" is not precisely documented, but it has been in use since the 19th century. It likely developed as a figurative expression from the more literal use of "reckon" (meaning to calculate or consider) in the context of making predictions, assumptions, or plans. It is still used in both formal and informal contexts to caution against overlooking important elements in decision-making.