IPA: /ʌnˈmɛmərəbli/
KK: /ʌnˈmɛmərəbli/
In a way that is not memorable or does not leave a lasting impression.
The movie was so dull that it ended unmemorably, leaving no impact on the audience.
Unmemorably → It is formed from "un-" (meaning not) and "memorable" (from Latin "memorabilis", meaning worthy of being remembered). The word "unmemorably" means in a way that is not worthy of being remembered or easily forgotten.
Think of 'not' ('un-') being combined with 'worthy of being remembered' ('memorabilis') — that's why unmemorably means something that is easily forgotten.