Of Definition
(military) NATO military rank code, equivalent to British army lieutenant and second lieutenant.
(military) NATO military rank code, equivalent to British army field marshal.
(usually in modal perfect constructions) Representing have or 've, chiefly in depictions of colloquial speech.
(UK) A regulatory body for a specified industry.
Origin of Of
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From Middle English of, from Old English of (“of, from"), an unstressed form of af, æf (“from, off, away"), from Proto-Germanic *ab (“from"), from Proto-Indo-European *hâ‚‚epo (“from, off, back"). Cognate with Scots of, af (“off, away"), West Frisian af, ôf (“off, away"), Dutch af (“off, from"), Low German af (“off, from"), German ab (“off, from"), Danish af (“of"), Swedish av (“of"), Icelandic af (“of"), Gothic 𐌰𐍆 (af, “of, from"); and with Latin ab (“of, from, by"). Compare off.
From Wiktionary
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Middle English from Old English apo- in Indo-European roots
From American Heritage Dictionary of the English Language, 5th Edition
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first syllable of office
From Wiktionary