A way to announce or say that some one is lying, untruthful, false, dishonest, mendacious, perfidious, deceitful, deceiving, deceptive, duplicitous, dissimulating, dissembling, double-dealing,
two-faced,
Janus-faced, guileful, underhand, disingenuous, crooked,
bent, sneaky, tricky, hollow-hearted, ect. Most of your best friends will likely say this to you.