On or during this present day. It can also be used as a noun to mean the current time period.
From Old English 'tō dæg' meaning 'on (this) day,' a combination of 'to' and 'day.' Over time the phrase fused into a single word. Similar compounds exist in other Germanic languages like German 'heute' ('this day').
Words like 'today,' 'tonight,' and 'tomorrow' started as little phrases that fused through constant use. The language literally glued 'to' and 'day' together because people said them so often. Grammar is full of these fast‑tracks carved by everyday speech.
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