IPA: /ˌɪn.fəˈlɪs.ɪ.təs.li/
KK: /ɪnˈfɛlɪsɪtəsli/
In a manner that is inappropriate or unsuitable, often leading to an unfortunate outcome.
He infelicitously commented on her appearance at the party, making everyone uncomfortable.
Infelicitously → It is formed from "in-" (meaning not), "felix" (from Latin, meaning happy or fortunate), and "-ous" (meaning full of) combined with the adverbial suffix "-ly". The word describes a manner that is not full of happiness or is unfortunate.
Think of 'not' ('in-') being combined with 'happy' ('felix') and 'full of' ('-ous') to remember that 'infelicitously' means in an unfortunate or unhappy manner.