(noun) - (1) A wanton wench that is ready to ride upon the men's backs, or else passively to be their rompstall. The word mutton, when applied to a woman, whether alone or as part of a compound epithet, seems always to have been opprobrious, as in Shakespeare's Two Gentlemen of Verona: "Ay, sir; I, a lost mutton, gave your letter to her, a laced mutton; and she, a laced mutton, gave me, a lost mutton, nothing for my labour." From rig, rigging, ready to bestride any inactive stallion, and give him a quickening spur.
--Frederick Elworthy's Specimens of English Dialects: Devonshire, 1879
(2) Rigmutton rumpstall, a wanton girl. West Country.
--Joseph Wright's English Dialect Dictionary, 1898-1905