happening, coming, made, or done quickly, without warning, or unexpectedly

a sudden attack.

occurring without transition from the previous form, state, etc.; abrupt

a sudden turn.

impetuous; rash.

Archaic . quickly made or provided.

Obsolete . unpremeditated.

Literary . suddenly.

Obsolete . an unexpected occasion or occurrence. ?

all of a sudden

without warning; unexpectedly; suddenly.

adjective

expected , slow

adjective

abrupt , accelerated , acute , expeditious , fast , flash , fleet , hasty , headlong , hurried , immediate , impetuous , impromptu , impulsive , out of the blue * , precipitant , precipitate , precipitous , quick , quickened , rapid , rash , rushing , spasmodic , speeded , subito , swift , unforeseen , unusual , imminent , instant , meteoric , speedy , spur-of-the-moment , surprising , uncontrolled , unexpected , unlocked for