Middle English couth "known, well-known; usual, customary," from Old English cuðe "known," past participle of cunnan "to know," less commonly "to have power to, to be able" (see can (v.1)).
As a past participle it died out 16c. with the emergence of could, but the old word was reborn 1896, with a new sense of "cultured, refined," as a back-formation from uncouth (q.v.). The Old English word forms the first element in the masc. proper name Cuthbert, which literally means "famous-bright."
Old English 1st & 3rd person singular present indicative of cunnan "to know," less commonly as an auxiliary, "to have power to, to be able," (also "to have carnal knowledge"), from Proto-Germanic *kunnjanan "to be mentally able, to have learned" (source also of Old Norse kenna "to become acquainted, try," Old Frisian kanna "to recognize, admit, know," German kennen "to know," Middle Dutch kennen "to know," Gothic kannjan "to make known"), from PIE root *gno- "to know."
It holds now only the third sense of "to know," that of "to know how to do something" (as opposed to "to know as a fact" and "to be acquainted with" something or someone). Also used in the sense of may, denoting mere permission. An Old English preterite-present verb, its original past participle, couth, survived only in negation (see uncouth), but compare could. The present participle has spun off with a deflected sense as cunning.
*gnō-, Proto-Indo-European root meaning "to know."
It forms all or part of: acknowledge; acquaint; agnostic; anagnorisis; astrognosy; can (v.1) "have power to, be able;" cognition; cognizance; con (n.2) "study;" connoisseur; could; couth; cunning; diagnosis; ennoble; gnome; (n.2) "short, pithy statement of general truth;" gnomic; gnomon; gnosis; gnostic; Gnostic; ignoble; ignorant; ignore; incognito; ken (n.1) "cognizance, intellectual view;" kenning; kith; know; knowledge; narrate; narration; nobility; noble; notice; notify; notion; notorious; physiognomy; prognosis; quaint; recognize; reconnaissance; reconnoiter; uncouth; Zend.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit jna- "know;" Avestan zainti- "knowledge," Old Persian xšnasatiy "he shall know;" Old Church Slavonic znati "recognizes," Russian znat "to know;" Latin gnoscere "get to know," nobilis "known, famous, noble;" Greek gignōskein "to know," gnōtos "known," gnōsis "knowledge, inquiry;" Old Irish gnath "known;" German kennen "to know," Gothic kannjan "to make known."
in writing, to indicate the common casual pronunciation of could have, by 1909.
surname, from the many Mortons on the map of England, literally "moor or marsh settlement." Morton's Fork (1759) is in reference to John Morton (c. 1420-1500), archbishop of Canterbury, who levied forced loans under Henry VII by arguing the obviously rich could afford to pay and those who were not were living frugally and thus had savings and could pay, too.