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reckon without (someone or something)
To fail to consider or anticipate someone or something as a factor in some decision, situation, or outcome. I thought we'd have enough money for a deposit on a house, but we reckoned without needing to pay for Sophie's trip to the emergency room. Too many companies reckon without the pitfalls that make insurance like this an absolute necessity.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
reckon without someone
to fail to think about someone. He thought he'd get away with his crime, but he reckoned without the FBI agents. He had thought he was gone for sure, but he had reckoned without the paramedics.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
To fail to consider or deal with someone or something; ignore someone or something: They thought it would be an easy victory, but they had reckoned without the determination of their enemy.
The American Heritage® Dictionary of Phrasal Verbs. Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.