Atrocious, enormous; excessively, exceedingly. Of the latter or adverbial use of the word Grose says, "It is an epithet, which, in the English vulgar language, is made to agree with every quality or thing; as, devilish bad, devilish good; devilish sick, devilish well; devilish sweet, devilish sour; devilish hot, devilish cold, &c."--Slang Dictionary.
A devilish knave! besides, the knave is handsome, young, and blithe; all those requisites are in him that delight.--Shakspeare.
Thy hair and beard are of a different die,
Short of one foot, distorted of one eye;
With all these tokens of a knave complete.
If thou art honest, thou'rt a devilish cheat.
Addison.