IPA: //ˈkɔːr.tʃəs.nəs//
KK: /ˈkɔr.t̬ə.əs.nəs/
The quality of being polite and respectful towards others.
Her courteousness made everyone feel welcome at the event.
Courteousness is formed from "courteous" (from Old French *cortois*, meaning polite or refined) and the suffix "-ness" (meaning the state or quality of). The word "courteousness" refers to the quality of being polite or showing good manners.
Think of the word 'courteous' which means polite, and remember that 'courteousness' is the state of being polite.