different between undersock vs underlock

English

under- +? sock

undersock (plural undersocks)

under- +? lock

underlock (plural underlocks)

A lock of wool hanging under the belly of a sheep.
- 1839, British Farmer's Magazine (volume 3, page 187)
  No cloggings, lumps of dirt, or underlocks should be suffered to be rolled up with the fleece, as such a practice is unfair to the purchaser, and by act of parliament there is a heavy penalty attached to those who are convicted of doing it.

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