IPA: /ˌɪmɪˈmɔːrɪəli/
KK: /ɪmɪˈmɔrɪəli/
In a way that has existed for a very long time, often beyond memory or recorded history.
The traditions of the village have been passed down immemorially through generations.
Immemorially → It is formed from "in-" (meaning not) and "memorialis" (from Latin "memoria", meaning memory). The word refers to something that is so old that it cannot be remembered, or beyond memory.
Think of something that is 'not' ('in-') within 'memory' ('memoria') — that's why immemorially refers to a time so distant that it is beyond memory.