Wiktionary
n. behavior that is intended to seem humble but comes across as fake and unflattering.
Usage examples of "false modesty".
His army was more mobile than any the city-dwellers could patch together, and he knew with no false modesty that he surpassed any southron general.
They are remarkable enough without my having to resort to false modesty.
Presidents liked to call it the People's House, to use the political voice of false modesty to describe a place for which some of them would have willingly run over the bodies of their own children, then say that it wasn't really all that big a thing.
When Lord Golden assured him that he knew such a statement was false modesty and that he would never dream of depriving the Buckkeep ladies of such a graceful partner, the man looked torn between disappointment at his dismissal and gratitude for the compliment.