Collocation

To me it means nothing more than "inequitable" + "blunder", with those words having their standard dictionary definitions.

Thanks a lot

Doesn't it mean a mistake that can't be forgiven?

Did you check "inequitable" in the dictionary? While the mistake might also be unforgivable, "inequitable" actually means "unfair".

Thanks,

Actually I was quoting my teacher's words on the meaning of this phrase inequitable blunder which he told it meant unforgivable mistake.

With due respect to your teacher, that is not actually what "inequitable" means.