定 places uprightness under a roof 宀: the household in order, everything in its place. From 'settled' came 'fixed', 'decided', and the adverb of certainty — what's settled is sure.
一定 'certainly' seals Chinese promises: 我一定来 'I will definitely come'. 决定 to decide, 定金 a deposit ('fixed money'). The app's own namesake line from《大学》continues: 知止而后有定 — knowing where to stop brings 定, stability.
Everything upright under the roof 宀 — SETTLED, DECIDED, CERTAIN.
淡定 'calm and settled' is the modern compliment for grace under pressure — the internet's favorite two-character prescription for every crisis.