Definition of "knuck"

There is a house in Cherry street, not far from Catherine Market […] . A little to the north of its door stands an old-time tree; and for many a year it has been known to the “crossmen” and “knucks” of the town as “Jack Circle's watering place” and “fence.”

1848, Ned Buntline, The Mysteries and Miseries of New York: A Story of Real Life, Part 1, New York: Benford & Co., page 33

A knuck in the front rank of a crowd desiring to steal a watch from the pocket of a gentleman standing on either side of him, first folds his arms across his breast; and pretending to be intensely looking at some object before him, stretches out the arm next his victim, […]

1859, George Washington Matsell, Vocabulum: Or, The Rogue's Lexicon (page 22)