mightn't /ˈmaɪt ənt/, the contraction of might not.
The vowel of not reduces to /ə/: /maɪt nɒt/ becomes /ˈmaɪt ənt/, an unstressed /ənt/ ending. The form is uncommon today rather than missing.
I mightn't have noticed it.
Hear it in a sentence
I mightn't be free tomorrow, so ring me first. Here the contraction keeps the polite, tentative feel of might not in a natural spoken line.
You mightn't like the answer, but you asked for it. A casual remark where the shorter form sits comfortably.
Journalists have used it recently too: reporting and opinion pieces in 2023 and 2024 include lines such as mightn't have been expected, showing the contraction survives in real, current writing.
Pronunciation trap
Mightn't is informal and especially British English, so it is not standard everywhere. Because it is informal and mainly British, it fits casual conversation, but it can sound out of place in formal writing or in American contexts, where dictionaries record the pronunciation /ˈmaɪt nt/ and speakers usually say the full form. Saying the full form /maɪt nɒt/ is never wrong, so in a sentence like I might not have time tomorrow, sorry the uncontracted form is the more natural choice.
Watch the ending: /ə nt/ is one light syllable, not a full /nɒt/. The o of not is gone entirely, and the vowel reduces to /ə/. Saying /ˈmaɪt nɒt/ in full instead is common and never wrong, so treat the contraction as an option, not a rule.