Regarding merkin it seems to be a variant of Malkin, which has an earlier entry in OED.
Origin: Probably a variant or alteration of another lexical item. Or perhaps from a proper name. Etymons: malkin n.; proper name Mary.
Etymology: Probably originally a variant of malkin n., or < a parallel pet-form of the female forename Mary (compare the Middle English surnames Marekin, Marykin).
Origin: From a proper name. Etymon: proper name Malkin.
Etymology: < the female forename Malkin < Mal , pet-form of the female forename Maud (compare Middle English forms Mald , Mold ; compare also similar derogatory use in Welsh of the equivalent name Mald ) + -KIN suffix; in later use (in sense 1c) sometimes reanalysed as < Mal , Mall , Malle , pet-form of the female forename Mary (compare MOLLn.2) + -KIN suffix. Compare use as a surname from the late 13th cent. With sense 1bcompare MERKIN n.1
Now arch. and regional.
a. A typical name (usually derogatory) for: a lower-class, untidy, or sluttish woman, esp. a servant or country girl. In Scotland: an awkward or ungainly young girl. †Also in various proverbial expressions, esp. there are more maids than Malkin (obsolete).
c1275 Lutel Soth Serm. (Calig.) 54 in R. Morris Old Eng. Misc. (1872) 188 Ne þeos prude ȝungemen Þat luuieþ malekin, And þeos prude maidenes Þat luuieþ Ianekin.
c1390 CHAUCER Man of Law’s Tale 30 It wol nat come agayn..Namoore than wol Malkyns maydenhede, Whan she hath lost it in hir wantownesse.
c1400 (▸a1376) LANGLAND Piers Plowman (Trin. Cambr. R.3.14) (1960) A. I.158 Ȝe ne haue no more meryt..Þanne malkyn of hire maidenhed, þat no man desiriþ.