IPA: /ˌɪm.ɪˈmɔːr.i.əl/
KK: /ɪˈmɛm.əˌrɪəl/
Existing for a very long time, often beyond the memory of people or recorded history.
The ancient trees in the forest are said to be immemorial, having stood there for centuries.
Immemorial is formed from "in-" (meaning not) and "memorialis" (from "memoria", meaning memory). The word describes something that is so old that it cannot be remembered or is beyond memory.
Think of something that is 'not' ('in-') within 'memory' ('memoria') — that's why immemorial refers to something so ancient that it is beyond recall.