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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
(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’.
Farlex Partner Idioms Dictionary © Farlex 2017