• (n.) Small splinters of stone inserted into the joints of coarse masonry.
Gar'ret·ing noun Small splinters of stone inserted into the joints of coarse masonry. Weale. 
Garreting is small splinters of stone, inserted in the joints of coarse masonry. They are stuck in after the work is built. Flint walls are very frequently garreted.
The insertion of small splinters of stone in the mortar joints before the mortar has firmly set.