Origin and meaning of "rackers"

I have read that Don Armado is one of those rackers of orthography who distort the written language when they convert it into speech. What is the origin of the word rackers? What does it mean?

Holofeners: "He draweth out the thread of his verbosity finer than the staple of his argument [...] such rackers of orthography, as to speak dout, fine, when he should said doubt; [...]."

If you'd spelt Holofernes right, it would have been easier to pick up that this is from Shakespeare. Holofernes, in Love's Labour Lost, is a pompous, self-satisfied schoolmaster, who takes it on himself to criticise other people's English. (Thank goodness we have none such on this site.) He means that Don Armado is torturing the language when he pronounces doubt 'dout', and debt, 'det'. Yes, these are the modern pronunciations. Even in the sixteenth century they were common; but it is possible, if you are a small-minded pedant with less education than you think, to say they are wrong, based on etymology. It would be a mistake to think Holofernes is meant as a serious character; much more so to rely on his views on English.

I would hazard a guess that it has something to do with the rack. NOAD defines rack (v.) as to "cause extreme physical or mental pain to; subject to extreme stress," which definition is obviously based on its subsidiary one, to "torture (someone) on the rack," which was "an instrument of torture consisting of a frame on which the victim was stretched by turning rollers to which the wrists and ankles were tied."