The OED has an interesting definition, Paul; I was fascinated to see that "mutton" is "goat" not only in South East Asia, but also in Australia:
b. Austral. and S. Asian. The flesh of goats, used as food. Also goat mutton.
1897 J. J. MURIF From Ocean to Ocean 57 No sheep beyond Oodnadatta either... The goat's flesh is called ‘mutton’. 1930 D. COTTRELL Earth Battle 125 She put..the damper and cold goat mutton on a newspaper. 1988 S. RUSHDIE Sameen Rushdie's Indian Cookery iii. 50 In subcontinental English, ‘mutton’ is the name for the meat of both goat and kid... Our mutton, or goat's meat, is leaner, more tender and delicatedly flavoured then lamb. 1999 Kathmandu Post (Electronic ed.) 15 Oct., The annual festival is the time when many families ensure steady supply of meat with a purchase of a goat. Mutton varieties are the most popular item in the Nepali menu during Dashain.
However a bit to the east a somewhat smaller island has another meaning:
2. a. A sheep, esp. one intended to be eaten. Chiefly N.Z. or humorous in later use.
?a1325 in W. Heuser Kildare-Gedichte (1904) 135 at on him send gees and henne, at oer geet and motune. a1393 GOWER Confessio Amantis (Fairf.) Prol. 1060 The Wolf in pes with the Moltoun. ?a1400 (a1338) R. MANNYNG Chron. (Petyt) II. 174 A bouke of a motoun. 1481 CAXTON tr. Godfrey of Boloyne clvii. 231 Oxen, Kyen, Motons and other vytaylles. 1565 LADY LOVAT in J. Fraser Polichron. (1905) 153 With twa mutowns yearly price of the pice thratin s. iiijd. 1615 G. SANDYS Relation of Iourney 37 Moldavia and Valachia do serve them with beeves and muttons. 1692 R. L'ESTRANGE Fables cccclxxxv. 461 The Sheep in this Fable was clearly too hard for the Two Doctors; and we find all those Reasonings to be true in the World, which the Mutton Alleges in the Fiction. 1749 E. SYNGE Let. 26 Sept. (1996) 172 Don't order a Mutton to be kill'd. I have but a few. 1795 W. COWPER Needless Alarm 81 A mutton, statelier than the rest, A ram, the ewes and wethers, sad, address'd. 1839 THACKERAY Legend St. Sophia of Kioff, A humble company of pious men, Like muttons in a pen. 1868 R. BROWNING Ring & Bk. II. IV. 7 How long, now, would the roughest marketman..Harass a mutton ere she made a mouth Or menaced biting? 1956 J. DARE Rouseabout Jane 171 It was not long before I was promoted to killing three muttons each week. 1988 C. GIDLEY Armada (BNC) 46 Leonora had had a mutton killed in anticipation of a family celebration.
The most general meaning is the meat of sheep:
a. The flesh of sheep, used as food.
c1300 St. Mary Magdalen (Laud) 344 in C. Horstmann Early S.-Eng. Legendary (1887) 472 Huy nomen with heom in heore schip..Gies and hennes..porc, motoun, and beof. c1450 in T. Austin Two 15th-cent. Cookery-bks. (1888) 72 Stwed Mutton: Take faire Mutton that hath ben roste..and mynce it faire. a1475 Liber Cocorum 46 Take fresshe brothe of motene clene. a1475 J. RUSSELL Bk. Nurture 533 in F. J. Furnivall Early Eng. Meals & Manners (1931) 36 Mustard is meete for brawne, beef, or powdred motoun. c1480 (a1400) St. Julian (Cambr.) 114 in W. M. Metcalfe Legends Saints Sc. Dial. (1896) I. 461 Sancte Julyane..In til his tyme wes na glotone, na wont wes nocht to ete motone. 1533 T. MORE Answere Poysened Bk. in Wks. 1059/1 Men bye bief or moten out of the bouchers shoppes. a1575 G. GASCOIGNE Hearbes in Posies 147 Fiue flocks of sheepe coulde scarce mainteine good mutton for his house. 1609 in Rothesay Town Council Rec. (1935) 166 Ane jeikat of muttoun. 1620 T. VENNER Via Recta iii. 50 Of Mutton..that is the best, which is of an yeere or two olde. 1711 SWIFT Jrnl. to Stella 19 Mar. (1948) I. 219 They..had a breast of mutton and a pint of wine. 1747 G. UNDERWOOD Let. 13 July. in A. P. Jenkins Corr. T. Secker (1991) 159 The provisions..were a Loin, a Rib, two Rumps and..two legs of Mutton. 1784 E. ALLEN Reason iii. §4. 137 To reward moral actions with a glass of wine or a shoulder of mutton, would be as inadequate, as to measure a triangle with sound. 1848 A. H. CLOUGH Bothie of Toper-Na-Fuosich V. 35 Racing home for the eight o'clock mutton. 1870 J. YEATS Nat. Hist. Commerce I. v. 49 Welsh sheep are small, but the mutton is renowned for the delicacy of its flavour. 1897 ‘H. S. MERRIMAN’ In Kedar's Tents x, The steaming dish of mutton and vegetables. 1922 V. WOOLF Jacob's Room iii. 53 She directed the maid to give Mr. Flanders a second helping of mutton. 1980 G. LORD Fortress viii. 69 The mutton was cooked brown and stringy.