of a sudden
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all of a sudden
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.
(all) of a suddensuddenly.
As a noun sudden is now found only in this phrase, but from the mid 16th century to the early 18th century it was in regular use in the sense ‘an unexpected danger or emergency’.