Probably originally a cavity, and the same word as dimple.
dimble (plural dimbles)
Part or all of this entry has been imported from the 1913 edition of Webster’s Dictionary, which is now free of copyright and hence in the public domain. The imported definitions may be significantly out of date, and any more recent senses may be completely missing.
(See the entry for dimble in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913.)