na-adjective, noun which may take the genitive case particle 'no', noun
outrageous; preposterous; scandalous; inexcusable
A strong yojijukugo used to condemn something as utterly unacceptable, beyond words, or morally outrageous. Often used in formal or written contexts to express strong disapproval.
彼の行動は言語道断だ。
His behavior is absolutely outrageous.
そんな言い訳は言語道断だ。
Such an excuse is utterly inexcusable.