IPA: /ˌʌnəˈkaʊntəbli/
KK: /ʌnəˈkaʊntəbli/
In a way that cannot be explained or understood; without a clear reason.
She unaccountably decided to leave the party early, leaving everyone confused.
Unaccountably → It is formed from "un-" (meaning not) and "accountable" (from Latin "accomptare", meaning to calculate or to count). The word means not able to be explained or justified, indicating a lack of accountability or reason.
Think of 'not' ('un-') being applied to 'counting' or 'calculating' ('accountable') — this helps you remember that 'unaccountably' means something that cannot be explained.