E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The part of the Court occupied by the lawyers.
To get a mans head into chancery is to get it under your arm, where you can pummel it as long as you like, and he cannot get it free without great difficulty. The allusion is to the long and exhausting nature of a Chancery suit. If a man once gets his head there, the lawyers punish him to their hearts content.
When I can perform my mile in eight minutes, or a little less, I feel as if I had old Times head in chancery.Holmes: Autocrat, chap. vii. p. 191.