In the Roman legal procedure of legis actio sacramento in rem ('legal assertion of claim'): the staff that was symbolically applied to the slave or item of property in order to give concrete expression to the plaintiff’s claim and the defendant’s counterclaim to the item. The etymology of vindicta is disputed (cf. most recently [1. 47 f.12]). The most probable connection appears to be with vim dicere ('to assert that one has - legally founded - power over the item').
1 A. Bürge, Römisches Privatrecht, 1999.