IPA: /ʌnˈfɔːɡətəbl/
KK: /ʌnˈfɔrɡɛtəbl/
Something that is so special or remarkable that it cannot be forgotten.
The concert was an unforgettable experience for everyone who attended.
Unforgettable → It is formed from "un-" (meaning not) and "forgettable" (from the root "forget" which comes from Old English "forgietan", meaning to lose memory of). The word "unforgettable" means something that cannot be forgotten or is impossible to lose memory of.
Think of something that is 'not' ('un-') able to be 'lost from memory' ('forgettable'). This helps you remember that 'unforgettable' describes something that stays in your mind.