IPA: /ˌʌnəʊˌstɛnˈteɪʃəs/
KK: /ʌnəˈstɛnˌteɪʃəs/
Not showy or flashy; simple and modest in appearance or behavior.
Her unostentatious style made her stand out in a crowd of flashy outfits.
Unostentatious → It is formed from "un-" (meaning not), "ostentatious" (from Latin "ostentare", meaning to show off or display). The word describes a quality of being not showy or pretentious, emphasizing simplicity and modesty.
Think of 'not' ('un-') being combined with 'showing off' ('ostentatious') to remember that 'unostentatious' means being modest and not flashy.