Jack o' the Clock
The figure which comes out to strike the hours on the bell of a clock. A contraction of Jaquemart (q.v.).
King Richard. Well, but what's o'clock? Buckingham. Upon the stroke of ten. K. R. Well, let it strike. R. Why let it strike? K. R. Because that, like a jack, thou keep's the stroke Betwixt thy begging and my mediation.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894