so bad, stupid, etc. it isn't true

so (something) it isn't true

Someone or something has so much of a particular characteristic or quality that it is hard to believe. Don't believe a word he says; that politician is so corrupt it isn't true!
See also: true
Farlex Dictionary of Idioms.

so ˌbad, ˌstupid, etc. it isn’t ˈtrue

(informal) used to emphasize that somebody/something is very bad, stupid, etc: His brother is so lazy it isn’t true!
See also: true
Farlex Partner Idioms Dictionary
See also:
  • so (something) it isn't true
  • believe (one's) ears
  • believe ears
  • believe one's ears
  • give (one) to believe
  • give somebody to believe/understand...
  • lead (one) to believe
  • lead somebody to believe
  • lead to believe
  • I do believe