Devilish means mischievously wicked in a sly, teasing way. A grin, or a trick that is plainly up to no good.
The word is far lighter than its root suggests, and a devilish glint in the eye is charming rather than sinister. It also works as an intensifier in older British English, so devilish difficult means extremely difficult, and that use now sounds distinctly period. Applied to a problem it means fiendishly hard. Its close relatives are impish, which is gentler, and diabolical, which in British speech usually just means very bad indeed.
Middle English
develissh
English, today
devilish
From Middle English develissh, develyssh, equivalent to devil + -ish. Cognate with stq düvelsk, düüwelsk (“devilish”), West Frisian duvelsk (“devilish”), Dutch duivels (“devilish”), German Low German düvelsk (“devilish”), German teuflisch (“devilish”), Danish djævelsk (“devilish”), Swedish djävulsk (“devilish”), Norwegian djevelsk (“devilish”).
Etymology from Wiktionary, CC BY-SA