fail on

fail (someone) on (something)

To give someone a failing grade on a school assignment. You clearly put zero effort into this essay—that's why I failed you on it.
See also: fail, on
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

fail someone on something

to give someone an unsatisfactory grade on an assignment or test. She failed us all on the math assignment. The teacher failed half the class on the assignment.
See also: fail, on
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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