yesterday, n. [
yes-ter-dey,
yěs-tər-dā]
-Though the requester correctly noted that
yesterday and
yeast have similar pronunciations and spellings, they are not in fact related.
Yesterday is first seen as a single word in a document from about 1250 CE, being spelled as
yisterdai. It was formed from the Old English compound noun
geostran dæg 'yesterday day' (c. 950), with
geostran being first recorded on its own around 725. While
geostran was originally sufficient on its own, over time it became absolutely paired with
dæg. Interestingly, though many of the cognates of Old English
geostran have equivalent definitions, several mean 'tomorrow' or both 'yesterday' and 'tomorrow': Middle Low German
gistern 'yesterday,' Middle Dutch
ghisteren 'yesterday,' Old Norse
gær 'tomorrow, yesterday,' Gothic
gistradagis 'tomorrow.' All of these stem from Proto-Germanic
gestra 'the other day,' which could refer to either before or after the present day, leading to the dichotomous definitions in the daughter languages. The Germanic root is thought to have descended from prehistoric Indo-European
ghes-, which also created Sanskrit
hyah, Avestan
zyo, Persian
di, Greek
khthes, Old Irish
indhe, and Welsh
doe, all meaning 'yesterday' or 'an indistinct past time.' Let's not forget Latin
heri 'yesterday' and
hesternus 'of yesterday,' the founder of French
hier and several other words for 'yesterday' in the Romance languages.