Learn English – Idiom or expression that would imply “someone is a habitual liar”

I suggest this old saying by Tucker Max "How were you supposed to know he would stoop so low? After all, the devil.......

Tucker Max - an American author and public speaker.

I would probably use the word diffident when someone is acquiescing to another's authority. The word connotes not just "simple" shyness, but also a lack of self-confidence and assertiveness.

Definition: (http://dictionary.reference.com/browse/diffident)

diffident adjective 1. lacking confidence in one's own ability, worth, or fitness; timid; shy.

Another option would be timid. It's a fairly common word and I don't think I need to define it here.

To more directly put across the point of submission to authority, you can consider submissive.

Definition: (http://dictionary.reference.com/browse/submissive)

submissive adjective 1. inclined or ready to submit or yield to the authority of another; unresistingly or humbly obedient: submissive servants. 2. marked by or indicating submission or an instance of yielding to the authority of another: a submissive reply.

Obedient is another (more positive) option. Again, a word in common usage, and I won't be defining it here (it's easy to look up).

The most negative options, generally used critically, are pushover and doormat. They are idiomatic nouns that describe a person who seriously lacks self-assertiveness and may be defined as follows:

Doormat (http://dictionary.reference.com/browse/doormat):

doormat

2. Slang One who submits meekly to domination or mistreatment by others.

Pushover (http://www.thefreedictionary.com/pushover)

push·o·ver

1. One that is easily defeated or taken advantage of.

In the same vein as the above two, there is another idiom, which I believe is peculiar to the US: milquetoast.

Milquetoast (http://dictionary.reference.com/browse/milquetoast)

milquetoast

noun, (sometimes initial capital letter)

1. a very timid, unassertive, spineless person, especially one who is easily dominated or intimidated: a milquetoast who's afraid to ask for a raise.

And, as included in the definition of that word spineless is another negative adjective for someone lacking self-assertiveness. Again, it's a highly critical word. Given that the spine supports your body in an upright posture, I believe you should be able to see how the metaphorical meaning comes about.