Falsus in uno, falsus in omnibus is a Latin maxim meaning "false in one thing, false in everything". At common law, it is the legal principle that a witness who testifies falsely about one matter is not credible to testify about any matter.

https://en.wikipedia.org/wiki/Falsus_in_uno,_falsus_in_omnib...

A phrase that eerily yet accurately sums up (or some would believe to sum up) lots of public discourse these days.