Unexpectedly and abruptly; suddenly. I was startled when the fire alarm went off all of a sudden.I thought Laura and I were friends, but all of a sudden, she stopped talking to me.
Entirely without warning, abruptly, as in All of a sudden the lights went out. In Shakespeare's day the common phrase was of a sudden, the word all being added in the late 1600s. Also see all at once, def. 2.