The night of this day; the time during the night that is coming soon.
It comes from Old English 'to niht', meaning 'this night' or 'on (the) night'. Over time, the phrase merged into the single word 'tonight'.
Words like 'tonight', 'today', and 'tomorrow' are actually old little time-phrases squashed together. English is full of these “time shortcuts” that show how often we talk about when things happen.
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