the word 'yesterday' has a direct lineage from old english 'geostran dæg', meaning 'yesterday day'. the first element, 'geostran', is related to old high german 'gestaron' and has proto-germanic roots, likely from a base meaning 'last' or 'past'. the second element, 'dæg', is the common germanic word for 'day'. over time, through regular sound changes and morphological fusion, the phrase condensed into the single lexical item 'yesterday', losing its transparent compound structure. it now functions as an indivisible adverb and noun referring specifically to the day immediately preceding the current one.

the word is a simplex unit derived from old english. it is not productively formed from smaller, independent morphemes in modern english.