1
general::
Phrase(s): count something against someone
to regard something in a negative way against someone. • I’m afraid we must count this against you as an unexcused absence. • Don’t count that last strike against the batter., Phrase(s): count against someone
[for something] to be held against someone; [for something] to weigh against someone. • I hope this mistake doesn’t count against me. • Don’t worry, it won’t count against you at all.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Be disadvantageous to, as in His earnings this year will count against his Social Security benefits. This idiom uses count in the sense of "make a reckoning," in this case negative. [Early 1900s]
American Heritage Idioms