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Old English þegn "military follower," also "servant, attendant," from Proto-Germanic *thegnas (cf. Old Saxon thegan "follower, warrior, boy," Old Norse þegn "thane, freeman," Old High German thegan, German Degen "thane, warrior, hero"), from PIE *tek-no- (cf. Sanskrit takman "descendant, child," Greek teknon "child"), from root *tek- "to beget, give birth to" (cf. Greek tekos "child, the young of animals," tokos "childbirth, offspring, produce of money, interest"). Also used in Old English for "disciple of Christ." Specific sense of "man who ranks between an earl and a freeman" is late 15c.
The modern spelling is from Scottish, where early 13c. it came to mean "chief of a clan, king's baron," and it has predominated in English probably due to the influence of "Macbeth;" normal orthographic changes from Old English ðegn would have produced Modern English *thain. Some historians now use thegn to distinguish Anglo-Saxon thanes from Scottish thanes.
in English history before the Norman Conquest (1066), a free retainer or lord, corresponding in its various grades to the post-Conquest baron and knight. The word is extant only once in the laws before the time of King Aethelstan (d. 939).