Etymology: Middle English trindel, from Old English tryndel, trendel circle, ring — more at trendle
2. : either of a pair of metal plates that are inserted between the backbone and boards of a rounded and backed book to force the backbone temporarily into a flat shape while the fore edge is being trimmed
II. \\-n(d)əl\ intransitive verb
Etymology: Middle English trindelen, probably from trindel trindle
Useful english dictionary. 2012.