on a sudden
• suddenly.
• "on a sudden a gleam of hope appeared."
all of a sudden
• suddenly.
• "I feel really tired all of a sudden"
Similar:
suddenly,
immediately,
instantaneously,
instantly,
in an instant,
straight away,
at once,
all at once,
promptly,
abruptly,
in a trice,
swiftly,
unexpectedly,
without warning,
without notice,
on the spur of the moment,
straight off,
out of the blue,
in a flash,
like a shot,
before you can say knife,
