courteous
adj.polite and respectful toward other people. You use this to describe someone who has very good manners.
adj.exhibiting politeness and respect in manner or behaviour. Often used to describe professional conduct or formal social interactions.
“The hotel staff were very courteous and helpful.”
“Even though the two politicians disagreed on every policy, they remained courteous to each other during the debate.”
“A courteous reply to a hostile email can often de-escalate a professional conflict more effectively than a defensive or equally aggressive response.”
Typically used as a predicative adjective after 'be' or 'remain', or as an attributive adjective before a noun.
From Middle English curteis, from Old French curteis (French courtois), from cort (“court”).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.