IPA: /ɪmˈprɒpənəs/
KK: /ɪmˈprɑːpərnəs/
The quality of being inappropriate or not suitable in behavior or actions.
His improperness at the formal event made everyone uncomfortable.
Improperness is formed from "im-" (meaning not) and "proper" (from Latin "proprius", meaning one's own or suitable) and the suffix "-ness" (meaning the state of being). The word refers to the state of not being suitable or appropriate.
Think of 'not' ('im-') being added to 'suitable' ('proper') to remember that 'improperness' means the state of being not suitable.