'yesterday' originates from Old English, specifically the word 'gēostran dæg', where 'gēostran' meant 'yesterday' and 'dæg' meant 'day'.

'gēostran dæg' transformed into the Middle English word 'yisterday', and eventually became the modern English word 'yesterday'.

Initially, it meant 'the day before today', and this meaning has largely remained the same in modern usage.