At fault. Not on the right track; doubtful whether right or wrong. Hounds are at fault when the scent is broken because the fox has jumped upon a wall, crossed a river, cut through a flock of sheep, or doubled like a hare.
In Geology, the break or displacement of a stratum of rock is called a fault.
(French, faute, Latin, fallo, to fail.)
“I am the youngest of that name, for fault of a worse.”
In fault. To blame.
“Is Antony or we in fault for this?”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894