IPA: /ˌʌnprɪˈpɒzɪŋ/
KK: /ʌnˌpriːpəˈzɛsɪŋ/
Something that is not very attractive or impressive; it lacks charm or appeal.
The restaurant had an unprepossessing exterior, but the food was excellent.
Unprepossessing → It is formed from "un-" (meaning not), "pre-" (meaning before), and "possessing" (from the root "possess" which comes from Latin "possidere", meaning to have or hold). The word describes something that does not create a favorable impression before being seen or experienced.
Think of something that is 'not' ('un-') creating a good impression 'before' ('pre-') you see it — that's why unprepossessing means it doesn't look appealing at first.