IPA: /ʌnˈkʌstəməri/
KK: /ʌnˈkʌstəmɛri/
Not usual or typical; different from what is commonly done or expected.
Wearing a suit to the beach is quite uncustomary.
Uncustomary → It is formed from "un-" (meaning not) and "customary" (from Old French *costumier*, meaning customary or usual). The word "uncustomary" means not usual or not in accordance with what is customary.
Think of 'not' ('un-') being added to 'customary' to remember that 'uncustomary' means something that is not usual.