Plural of 'dee,' the letter 'D' or the sound it makes, or the Roman numeral 500.
From the name of the letter D, which comes from Phoenician 'daleth' (door). The plural 'dees' has been used since medieval times when spelling out words letter by letter.
The letter D's name 'dee' comes from 'door' in Phoenician—a linguistic fossil showing how ancient speakers noticed the letter shape looked like a door, and that metaphor survived 3,000 years of language evolution.
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