used to mean a crack or a split.

chink (n.1)

"a split, crack," 1530s, with unetymological -k + Middle English chine (and replacing this word) "fissure, narrow valley," from Old English cinu, cine "fissure," which is related to cinan "to crack, split, gape," from Proto-Germanic *kino-(source also of Old Saxon and Old High German kinan, Gothic uskeinan, German keimen "to germinate;" Middle Dutch kene, Old Saxon kin, German Keim "germ"). The connection being in the notion of bursting open.

etymonline.com/word/chink#etymonline_v_11273