IPA: //ʌnˈsɛksɪst//
KK: /ʌnˈsɛksɪst/
The least attractive or appealing in a sexual or romantic way.
He wore the unsexiest outfit to the party, making everyone laugh.
Unsexiest → It is formed from "un-" (meaning not) and "sexy" (from Latin *sexus*, meaning sex or gender, combined with the modern English suffix "-y" which denotes a characteristic). The word "unsexiest" means not possessing the quality of being sexy, or the least sexy.
Think of 'not' ('un-') and 'sexiness' ('sexy') to remember that 'unsexiest' describes something that is the least attractive or appealing.
No commonly confused words.