court + -eous

royal court: 王室に合う振る舞いから

source: oc lion GIF - Find & Share on GIPHY

polite, respectful, or considerate in manner.: やり方が礼儀正しい、丁寧な、思いやりのある

〈人･行為などが〉 【他人に対して】礼儀正しい, 思いやりがある, 丁重な «to, with» (↔ discourteous; → polite)

courteously | ˈkərdēəslē | adverb

courteousness | ˈkərdēəsnəs | noun

Middle English (meaning ‘having manners fit for a royal court’): from Old French corteis, based on Latin cohors ‘yard, retinue’ (see court). The change in the ending in the 16th century was due to association with words ending in -eous.