How is someone who is mendacious?

A mendacious person is one who tells lies habitually and intentionally. Don't get stuck at the water cooler or bus stop next to someone you consider mendacious!

People may tell "white lies" if they forgot your birthday or really don't like your new haircut, but if you catch someone intentionally manipulating you with a falsehood, that person is just plain mendacious. So think of the most deceptive, insincere, perfidious, duplicitous, false person you've ever met, and then add the word mendacious to that list.