Sudden

1. Happening quickly and with little or no warning.
2. (obsolete) Hastily prepared or employed; quick; rapid.
3. (obsolete) Hasty; violent; rash; precipitate.

(happening quickly): abrupt, precipitous, subitaneous

(hasty, rash): hotheaded, impetuous, impulsive; reckless

(happening quickly): gradual

(all): unsudden