IPA: //ˈteɪs.ləs.li//
KK: /ˈteɪsˌlɛsli/
In a way that lacks flavor, style, or good taste.
The decor was tastelessly designed, with clashing colors and cheap materials.
Tastelessly → It is formed from "taste" (from Old French *taster*, meaning to perceive flavor) and "-less" (meaning without) and "-ly" (meaning in a manner). The word "tastelessly" means in a manner that lacks flavor or interest.
Think of 'taste' and remember that 'tastelessly' means lacking flavor or interest.