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.