IPA: /ˌʌnɪnˈlaɪtənɪŋ/
KK: /ʌnɪnˈlaɪtənɪŋ/
Not providing useful information or insight; lacking in clarity or understanding.
The lecture was quite unenlightening, leaving the students more confused than before.
Unenlightening → It is formed from "un-" (meaning not) and "enlighten" (from Old English "inleohte", meaning to give light or knowledge) and the suffix "-ing" (indicating the action or process). The word means the process of not providing light or knowledge, hence lacking enlightenment.
Think of 'not' ('un-') giving 'light' or 'knowledge' ('enlighten') — that's why unenlightening means something that does not provide understanding.