Mammock is an obscure word of indefinite origin. It means to break or tear into fragments or shreds. As a noun, it refers to the fragments, shreds, or scraps themselves.
In Ivanhoe, Gurth and Wamba react to the news that their friends — who are imprisoned in a castle — face possible execution:
"Then, by St Thomas of Canterbury," replied Gurth, "we will have the castle, should we tear it down with our hands!"
"We have nothing else to tear it with," replied Wamba; "but mine are scarce fit to make mammocks of freestone and mortar."