Chapter Jailer to Jar of J by Webster's Dictionary (1913 Edition)

2. Clash of interest or opinions; collision; discord; debate; slight disagreement.

And yet his peace is but continual jar.

Spenser.

Cease, cease these jars, and rest your minds in peace.

Shak.

3. A regular vibration, as of a pendulum.

I love thee not a jar of the clock.

Shak.

4. pl. In deep well boring, a device resembling two long chain links, for connecting a percussion drill to the rod or rope which works it, so that the drill is driven down by impact and is jerked loose when jammed.