Plural of yesterday; refers to multiple past days or the general concept of the past.
From Old English 'geostran dæg' (yesterday) or 'geostra dæg.' Compound of 'geostra' (of yesterday, formerly) and 'dæg' (day). The word structure mirrors how we speak of time as collections of days.
Using 'yesterdays' as a plural is poetic—it suggests that the past isn't just one moment but many accumulated days. Bob Dylan's 'All the Tired Horses' repeats 'All the tired horses in the sun' against 'yesterdays.' It's melancholic because you're claiming multiple pasts.
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