A liar is simply someone who tells a lie–and a lie is any statement that isn't true. A lie, in other words, is a deliberate attempt to deceive, and if that is someone's goal, then that person is a liar.
Why is it wrong to tell a lie, even if it doesn't seem to hurt anyone? One reason–which you should take very seriously–is because God commands us to tell the truth. One of the Ten Commandments states, "You shall not give false testimony against your neighbor" (Exodus 20:16). The Bible also says, "each of you must put off falsehood and speak truthfully to his neighbor" (Ephesians 4:25).
But another reason is because a lie always–without exception–hurts someone. How do you know what impact your words might have on someone else? The answer is, you don't. Your example also hurts others; do you honestly want your children (for instance) to grow up thinking it doesn't matter whether or not they tell the truth?
But most of all, a lie always hurts the person who tells it. It takes the moral edge off you, making you less concerned about God's truth and His will for your life. In addition, others will eventually realize that you can't be trusted.
Don't compromise on the truth. Instead, commit your life to Christ, and make it your goal to serve Him in everything–including your speech.