The actual value or rent of a tenement, and not that modified form on which the rates and taxes are usually levied. (Saxon, raecan, to stretch; Dutch, racken.)
“A rent which is equivalent, or nearly equivalent in amount, to the full annual value of the land, is a rack-rent.” —Encyclopaedia Britannica, vol. xx. p. 403.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894