adj. 25B6; adjective
his diabolical skill : DEVILISH, fiendish, satanic, demonic, demoniacal, hellish, infernal, evil, wicked, ungodly, unholy.
(informal) a diabolical performance : VERY BAD, dreadful, awful, terrible, disgraceful, shameful, lamentable, deplorable, appalling, atrocious; inferior, substandard, unsatisfactory, inadequate, second-rate, third-rate, shoddy, inept; informal crummy, dire, dismal, God-awful, abysmal, rotten, pathetic, pitiful, lousy; Brit. informal duff, rubbish, ropy.
(informal) a diabolical liberty : EXTREME, excessive, undue, inordinate, immoderate, unconscionable, outrageous; intolerable, unacceptable, unreasonable, unjustifiable, unwarrantable, inexcusable, unpardonable.