IPA: /ˌʌn.ədˈvɜːr.taɪzd/
KK: /ʌnˈæd.vɚˌtaɪzd/
Something that has not been promoted or made known to the public.
The store had an unadvertised sale that surprised many customers.
Unadvertised → It is formed from "un-" (meaning not) and "advertised" (from Latin "advertere", meaning to turn towards or direct attention to). The word "unadvertised" means not directed attention to or not made known publicly.
Think of 'not' ('un-') making something 'known' or 'directed attention to' ('advertised'). This helps you remember that unadvertised means something that hasn't been made known.