early 12c., from Old French chancelier (12c.), from Late Latin cancellarius "keeper of the barrier, secretary, usher of a law court," so called because he worked behind a lattice (Latin cancellus) at a basilica or law court (see chancel).
In the Roman Empire, a sort of court usher who stood at the latticed railing enclosing the judgment seat to keep the crowd out and admit those entitled to enter. The post gradually gained importance in the Western kingdoms as an intermediary between the petitioners and the judges as a notary or scribe. In England eventually he prepared all important crown documents and became keeper of the great seal and highest judicial officer of the crown. A variant form, canceler, existed in Old English, from Old North French, but was replaced by this central French form.