Idiom or expression that would imply "someone is a habitual liar"

Solution 1:

I've often heard "lies like a rug"

From McGraw-Hill Dictionary of American Idioms and Phrasal Verbs:

to tell lies shamelessly. He says he didn't take the money, but he's lying like a rug. I don't believe her. She lies like a rug

Solution 2:

I would use "pathological liar" which is broadly used to describe someone who habitually lies. The Wikipedia article on Pathological lying explains:

Pathological lying (also called pseudologia fantastica and mythomania) is a behavior of habitual or compulsive lying. It was first described in the medical literature in 1891 by Anton Delbrueck. Although it is a controversial topic, pathological lying has been defined as "falsification entirely disproportionate to any discernible end in view, may be extensive and very complicated, and may manifest over a period of years or even a lifetime". The individual may be aware they are lying, or may believe they are telling the truth. Sometimes however, the individual may be lying to make their life seem more exciting when in reality they believe their life is unpleasant or boring.

I heard this term used very often. Dictionary.com has the following definition:

a person who tells lies frequently, with no rational motive for doing so.

Solution 3:

Sounds like the person in question is full of shit!

This doesn't necessarily imply the lying is habitual, although you can modify it rather easily with any word indicating frequency:

Yea I know Bob said that, but Bob is usually full of shit.

Or just use it as is:

Bob said that? He's full of shit.

You can use "full of it" to avoid using "shit" in more polite contexts.

Other more polite forms include full of crap, full of bull, or full of baloney (thanks @DamianYerrick).

Solution 4:

"She lies like she breathes" would be the common idiom that comes to mind for me.

Interestingly, I can't find any references online that directly discuss or define this idiom, but it is common as an American idiom, implying that lying is as easy and essential to a given person as breathing, with the additional insinuation that (like breathing) they just lie all the time.

Googling the phrase produces a number of book, article headlines ('The Man Who Lies Like He Breathes Calls President Obama’s Campaign “Disgraceful”', http://www.politicalgarbagechute.com/the-man-who-lies-like-he-breathes-calls-president-obamas-campaign-disgraceful/), and excerpts from comment sections where this phrase is used.

I did see an alternate, "you can tell he's lying because he's breathing," but that seems more unwieldy and more literal.

Solution 5:

Not specific to lying, but I'm fond of the word inveterate:

Having a particular habit, activity, or interest that is long-established and unlikely to change

From ODO.

So inveterate liar does genuinely mean someone who lies out of habit.