Synonyms for Impermissible:

adj.

forbidden (adjective)
proscribed, verboten, taboo.
unspeakable (adjective)
unmentionable.

n.

allow. impermissible (noun)
inadmissible, unmentionable, out, untouchable, tabu, taboo, proscribed, prohibited, verboten, forbidden.

Other synonyms:

Other relevant words:
allow.