One of the mysteries of the English language finally explained.
Any of several kinds of North American oak having hard close-grained durable wood much used for posts, sleepers, etc., especially: (a) Quercus stellata, found in sandy soils of the central and eastern United States (also called iron-oak); (b) (in full "swamp post oak") Q. lyrata, of river swamps in the southern United States (also called overcup oak).
Late 18th century; earliest use found in Bernard Romans (c1720–?1784), cartographer and revolutionary army officer in America. From post + oak.
post oak/ˈpəʊst əʊk/
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