noun

• (by extension) A dull or uninteresting appearance or situation, unremarkable.
  "The slimy little causeway had dropped into the river by a slow process of suicide, and two or three stumps of piles and a rusty iron mooring-ring were all that remained of the departed Break-Neck glories. [...] [T]hrough three-fourths of its rising tides the dirty indecorous drab of a river would come solitarily oozing and lapping at the rusty ring, [...]"

adj

• (by extension) Particularly of color: dull, uninteresting.
  "[T]he man was about fifty-two—had a ſmall cane under his arm—was dreſs'd in a dark drab-colour'd coat, waiſtcoat, and breeches, which ſeem'd to have seen ſome years ſervice—they were ſtill clean, and there was a little air of frugal propretè throughout him."

noun

• (dated) A dirty or untidy woman; a slattern.
  "[C]ertainly thou deſireſt but thy right, that canſt read a rhetorique, or logique lecture to Hecuba in the art of raving, and inſtruct Tiſiphone herſelfe in her owne gnaſhing language. Other he, or ſhe, drabs of the curſteſt or vengeableſt rankes, are but dipped or dyed in the art; not ſuch a belldam in the whole kingdome of frogges, as thy croking, and moſt clamorous ſelfe."
• (dated) A promiscuous woman, a slut; a prostitute.
  "Take heed to false harlots, and more, ye wot what. / If noise ye heare, / Looke all be cleare: / Least drabs doe noie thee, / And theeues destroie thee."

verb

• (intransitive, obsolete) To consort with prostitutes; to whore.
  "You may ſay, you ſaw him at ſuch a time, marke you mee, / At game, or drincking, ſwearing, or drabbing, / You may go ſo farre."