A doss or footstool made of hesg (sedge or rushes).
“Hassocks should be gotten in the fens, and laid at the foot of the
said bank ... where need required.” —
“The knees and hassocks are well-nigh divorced.” Cowper.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Hassock from Infoplease: