Definition - The version of the Anglian dialect of Old English that was spoken in the Anglo-Saxon kingdom of Mercia. Modern Standard English descended from this dialect.
Note: According to the OED: "Although the political limits of Mercia were different at different periods, it may broadly be said to have occupied the middle of South Britain, between Deira on the north and Wessex on the south, and between Wales on the west and Lindsey and East Anglia on the east."

Oxford English Dictionary -
The term's first citation is from 1655:
"Wolphere, the Mercian King."
(Fuller Ch. Hist. ii. 89)

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