IPA: /ˈprɒpərɪst/
KK: /ˈprɒpərɪst/
The highest degree of being suitable, correct, or appropriate for a particular situation or purpose.
She always knows the properest way to address a formal audience.
Comparative: proper
Superlative: properest
Properest → It is formed from "proper" (from Old French *propre*, meaning one's own or suitable) and the superlative suffix "-est" (meaning most). The word "properest" means most suitable or most appropriate.
Think of something that is 'suitable' ('proper') and remember that 'properest' means the most suitable.