A term coined by Demetri Martin a stand up comedian. Its his form of LOL since he never has laughed out loud at his computer, he just LQTM or laughs quietly to myself.
Tina: LOL, you're so funny.
Rachel: LQTM, I know, I know.
Tina: WTF is LQTM?
Rachel: Laughing quietly to myself. I tend to be more truthful than others.
Means "laugh quietly to myself" originally used by Demetri Martin in a stand up comedy, used to replace "lol" to really describe what your doing when you say "lol".
Andy: "dude i got this huge History test tomorrow and im gonna totally fail"