九 began as a picture of a bent arm and hand reaching, hooking around — the elbow's curve gives it that hooking final stroke. It was borrowed for the sound 'nine', the largest single digit.
Nine is the number of the emperor and of eternity: it sounds like 久 (jiǔ, 'long-lasting'). The Forbidden City was said to have 9,999 rooms; imperial robes bore nine dragons. Lovers give gifts in nines to wish their love 天长地久 — 'as long as heaven and earth'.
A bent arm hooking around — the curve of NINE, the biggest single digit.
九 sounds like 久 ('long-lasting'), so it means eternity. It was the emperor's number — imperial gates had nine rows of nine golden studs, 81 in all.