IPA: /ˈsteɪd.nəs/
KK: /steɪd.nəs/
The quality of being serious, calm, and respectable, often in a way that is dull or unexciting.
Her staidness made her the perfect candidate for the formal event, where excitement was not the goal.
Staidness is formed from "staid" (from Middle English "stayed", meaning settled or fixed) and the suffix "-ness" (meaning the state or quality of). The word describes the quality of being settled or serious in demeanor.
Think of someone who is 'settled' and 'fixed' in their demeanor, which helps you remember that 'staidness' refers to a serious and steady quality.