IPA: /ˌsɛmɪˈpɒpjʊlə/
KK: /ˌsɛmɪˈpɑpjələr/
Having a moderate level of popularity, not fully popular but more than just a little known.
The band became semipopular after their second album was released.
Semipopular → It is formed from "semi-" (meaning half) and "popular" (from Latin "popularis", meaning of the people). The word "semipopular" refers to something that is half or partially popular among people.
Think of 'semi-' meaning half, and 'popular' meaning of the people — so semipopular describes something that is half liked or accepted by people.