Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
infestīvĭter, adv., v. infestivus fin.
in-festīvus, a, um, adj., not agreeable, not pleasant, inelegant (post-class.): subagresti ingenio et infestivo, Gell. 1, 5, 3.
Adv.: infestīvĭter, inelegantly: cetera vertit non infestiviter, i. e. quite elegantly, Gell. 9, 9, 9.