From the second edition (1989):
polymath, n. (a.)
(ˈpɒlɪmæθ) Also 7 polumathe. [ad. Gr. πολυµαθής having learnt much, f. πολυ- much + µαθ-, stem of µανθάνειν to learn. So F. polymathe.]

a. A person of much or varied learning; one acquainted with various subjects of study.

1621 Burton Anat. Mel. Democr. to Rdr. (1676) 4/2 To be thought and held Polumathes and Polyhistors. a1840 Moore Devil among Schol. 7 The Polymaths and Polyhistors, Polyglots and all their sisters. 1855 M. Pattison Ess. I. 290 He belongs to the class which German writers‥have denominated ‘Polymaths’. 1897 O. Smeaton Smollett ii. 30 One of the last of the mighty Scots polymaths.

b. attrib. or adj. Very learned. as adj.

1881 Athenæum 31 Sept. 300/3 [His] literary criticism‥is generally judicious and free from ‘polymath terminology’. 1893 Jrnl. Educ. 1 Dec. 657/1 A polymath headmaster. 1919 T. S. Eliot Poems, The masters of the subtle schools are controversial, polymath. 1959 New Scientist 15 Oct. 712/2 Polymath historians, who command the language and literature and science of the period. 1976 Nature 16 Sept. 261/3 The book is polymath, up-to-date and non-mathematical.

So polyˈmathic a., pertaining to a polymath, characterized by varied learning; †poˈlymathist = polymath; poˈlymathy [ad. Gr. πολυµαθία], much or varied learning, acquaintance with many branches of knowledge.

1828 Webster, *Polymathic, pertaining to polymathy. 1849 Otté tr. Humboldt's Cosmos II. 541 Necessity for a certain amount of polymathic learning. 1976 Publishers Weekly 16 Feb. 82/2 Crichton's polymathic talent has settled on another exotic site for a fanciful story. 1976 S. Hynes Auden Generation viii. 259 A synthesizing, diagnostic, polymathic mind, given to schematizing knowledge and history in elaborate diagrams. 1621 Bp. R. Montagu Diatribæ 322 An Atlas of Learning, the only *Polymathist of the World. c1645 Howell Lett. (1650) III. viii. 13 Polymathists, that stand poring‥upon a moth-eaten Author. 1642 Hartlib Ref. Schools 53 That high, and excellent learning, which men, for the large extent of it, call *Polymathie. 1865 Grote Plato I. ii. 88 Aristotle‥exhibits‥much of that polymathy which he transmitted to the Peripatetics generally. 1895 Beneke Comparetti's Vergil in Mid. Ages 224 The tendency‥of the scholars of the time,‥was entirely towards polymathy.