No great shakes. Nothing extraordinary; no such mighty bargain. The reference is to shingle for the roof of shanties, or to stubble left after harvest for the poor.
“The cabin itself is quite like that of the modern settlers, but the shingles, called shakes, ... make the wood roof unique.” —Harper's Weekly, July 18th, 1891, p. 534.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894