Definition of yesterday adjective from the Oxford Advanced American Dictionary

 

yesterday

 adjective
adjective
NAmE//ˈyɛstərˌdeɪ//
 
, NAmE//ˈyɛstərdi//
 
[not before noun](informal)(often humorous)
 
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no longer fashionable or new E-mail—that's so yesterday! PCs are so yesterday.
See the Oxford Advanced Learner's Dictionary entry: yesterday