definitely (adv.)youdaoicibaDictYouDict
1580s, from definite + -ly (2). As a colloquial emphatic word, attested by 1931.
indefinitely (adv.)youdaoicibaDictYouDict
early 15c.; see indefinite + -ly (2).
infinitely (adv.)youdaoicibaDictYouDict
early 15c., from infinite + -ly (2).