Where does the phrase “dead simple” originate?
According to Wiktionary, the phrase 'dead on' means 'very accurate' or 'exactly at'. This is also how I have used the phrase.
But why? What's the connection there?
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As per Etymonline:
I'm sure that someone with OED access can provide a complete answer with some citations.
If you consider someone shooting on a mark (at a target), then dead on could be considered an elision of dead on the mark. In other words, completely on target. A similar expression would be dead on the money.
According to one online source:
A few more details about the word dead in this and similar phrases is found in this ELU question.