Alexander Schmidt, Shakespeare Lexicon and Quotation Dictionary

A, a corruption of different particles and formative syllables; 1) being a prefix to many words; cf. Abase, Abashed, Abed etc.
2) preceding gerunds (most M. Edd. making use of the hyphen): “go a bat-fowling,” Tp. II, 1, 185. “sat a billing,” Ven. 366. “we'll a birding,” Wiv. III, 3, 247. “goes a birding,” III, 5, 46. III, 5, 46 “he's a birding,” IV, 2, 8. “lie a bleeding,” Rom. III, 1, 194. “fell a bleeding,” Merch. II, 5, 25. “are a breeding,” LLL I, 1, 97. “a brewing,” Merch. II, 5, 17. “falls a capering,” Merch. I, 2, 66. “a coming,” LLL V, 2, 589. “fall a cursing,” Hml. II, 2, 615. “it was a doing,” Cor. IV, 2, 5; cf. “as long a doing,” R3 III, 6, 7. “fell a doting,” Sonn. 20, 10. “go a ducking,” Ant. III, 7, 65. “a dying,” R2 II, 1, 90. “a feasting,” Wiv. II, 3, 92. “a going,” H8 I, 3, 50. “so long a growing,” R3 II, 4, 19. “was a hanging thee,” Lr. V, 3, 274. “fell a hooting,” LLL IV, 2, 61. “I would have him nine years a killing,” Oth IV, 1, 188. “a making,” Mcb. III, 4, 34. Hml. I, 3, 119. “a chime a mending,” Troil. I, 3, 159. “still a repairing,” LLL III, 193. “a ripening,” H8 III, 2, 357. “a rolling,” V, 3, 104. “set a shaking,” Lucr. 452. “fell a shouting,” Caes. I, 2, 223. “seems a sleeping,” Tim. I, 2, 68. “at gaming, a swearing,” Hml. III, 3, 91 (Ff om.). “she has been too long a talking of,” Ado III, 2, 107. “fell a turning,” Pilgr. 100. Pilgr. 100 “set me a weeping,” H4B II, 4, 301. “comes a wooing,” Shr. III, 1, 35. Oth. III, 3, 71.
3) before substantives; frequently changed to o', of and on, by M. Edd. Qq and Ff have almost always five a clock etc. (f. i. Ado III, 4, 52. H4A I, 2, 139. II, 1, 36.), M. Edd. throughout o' clock (cf. Clock). The same liberty they have taken with most of the following passages, in which a is supported by all or at least by the most authentic old texts:
a) a for of: “a mornings,” Ado III, 2, 42. “a days,” H4B II, 4, 251. Tim. IV, 3, 294. “a nights,” Tw. I, 3, 5. Tim. IV, 3, 292. Caes. I, 2, 193. II, 2, 116. “light a love,” Ado III, 4, 47. “cloth a gold,” III, 4, 19. “issue a my body,” Alls I, 3, 27. “out a friends,” Alls I, 3, 27 “a purifying a the song,” Alls I, 3, 27 “take leave a the king,” II, 4, 49. “our Isbels a the country and our Isbels a the court,” III, 2, 14. III, 2, 14 “out a the band,” IV, 3, 227. “no more a that,” IV, 2, 13. “a crow a the same nest,” IV, 3, 319. “a commoner a the camp,” V, 3, 194. “all the spots a the world,” V, 3, 206. “what dish a poison,” Tw. II, 5, 123. “inns a court,” H4B III, 2, 14 (Ff of). “John a Gaunt,” R2 I, 3, 76. H4B III, 2, 49. 344 (Ff of). “the sweet a the night,” V, 3, 53 (Ff of). “be a good cheer,” H5 II, 3, 19. “body a me,” H8 V, 2, 22. “were a my mind,” Troil II, 3, 225 (Q of). “loads a gravel,” V, 1, 22. “the sink a the body,” Cor. I, 1, 126; cf. I, 6, 47. II, 3, 79. V, 6, 83. V, 6, 83 V, 6, 83 V, 6, 83 “yond coin a the Capitol,” V, 4, 1. “time out a mind,” Rom. I, 4, 69; cf. out a door and “out a doors,” Err. II, 1, 11. H4B II, 4, 229. Cor. I, 3, 120. Hml. II, 1, 99. “the maid is fair, a the youngest for a bride,” Tim. I, 1, 126. “what time a day is it?” Tim. I, 1, 126 the heels a the ass, 282 etc.
b) for on: “a Monday,” Hml. II, 2, 406. “a Wednesday,” H4A V, 1, 138. Cor. I, 3, 64. “a Thursday,” H4A II, 4, 74 (Ff on). H4B II, 4, 298 (Ff on). Rom. III, 4, 20. III, 5, 162. “a Friday,” Troil. I, 1, 78 (Ff on). “a Sunday,” Shr. II, 318. “a Sundays,” Hml. IV, 5, 182. “I love a ballad in print a life,” Wint. IV, 4, 264. “a horseback,” H4A II, 3, 104. II, 4, 378. II, 4, 378 “a my word,” Shr. I, 2, 108. H4B II, 4, 190 (Ff on). Cor. I, 3, 62. Rom. I, 1, 1 (Qq on). “stand a tiptoe,” H5 IV, 3, 42. “heaved a high,” R3 IV, 4, 86, cf. “look up a height,” Lr. IV, 6, 58. “a my troth,” Cor. I, 3, 63. “a plague a both your houses,” Rom. III, 1, 93. Rom. III, 1, 93 “a pox a drowning,” Oth. I, 3, 366. “a conscience,” Per. IV, 2, 23.
c) for in: “a God's name,” Shr. I, 2, 195. IV, 5, 1. R2 II, 1, 251 (Ff o'). III, 3, 146 (Ff o'). H6A I, 2, 102. H6B II, 3, 54. IV, 7, 115. H8 II, 1, 78. “a this fashion,” Alls II, 3, 265. Hml. V, 1, 218 (Ff o'). “torn a pieces,” H8 V, 4, 80. “I'll see the church a your back,” Shr. V, 1, 5. “kept a coil,” Alls II, 1, 27.
Even this a before vowels sometimes changed to an: “set an edge,” Wint. IV, 3, 7. H4A III, 1, 133. “stand an end,” Hml. I, 5, 19. III, 4, 122 (in H6B III, 2, 318 and R3 I, 3, 304 Ff an end, Qq on end). “an hungry,” Cor. I, 1, 209 (a solecism formed in derision by Coriolanus). an't == on't, i. e. of it, Hml. V, 1, 26 (the gravedigger's speech).