Wiktionary
impermissibly
adv. (context chiefly legal English) In a way or to an extent that is not permissible.
WordNet
impermissibly
adv. not permissibly; "the radon level in the basement was impermissibly high" [ant: permissibly]
Usage examples of "impermissibly".
The intensity, the lyricism, the romanticism of love in these last two cases has struck some readers as impermissibly old-fashioned.