What is Bill Of Indemnity?

Legal Definition
In English law. An act of parliament, passed every session until 1869, but discontinued in and after that year, as having been rendered unnecessary by the passing of the promissory oaths act, 1868, for the relief of those who have unwittingly or unavoidably neglected to take the necessary oaths, etc., required for the purpose of qualifying them to hold their respective offices. Wharton.
-- Black's Law Dictionary
Legal Definition
A statute relieving officers from the failure to take their official oaths.
-- Ballentine's Law Dictionary