Na-adjective (keiyodoshi), Noun which may take the genitive case particle 'no', Noun
outrageous; preposterous; scandalous; inexcusable; absurd; execrable
Yojijukugo (four character compound)
きみの言語道断な行いを恥ずかしいと思う。I am ashamed of your despicable deed.
Also written 言語同断【ごんごどうだん】