What is it's meaning? I sometimes do this subconsciously and at other times deliberately and I often do that when I'm alone (at the computer desk for instance) or when I'm talking with others while being seated. I hunch over a little and while I thrust my elbow out to the side, I place my left clenched hand on the left thigh while I am seated. I also do not clench my thumb while doing this. It's always the left hand on my left thigh while I am seated.
To give you a better cultural context, I'm an East Asian Indian, but grew up in the United States. I'm 42 years old. By the way, I don't think this purely an intrapersonal issue as I do this while talking to others also. I'm asking this because I think I'm getting negative reactions to this posture as my conversational partners quickly glances at my left arm as if they are not pleased. I find this posture comfortable. So that's probably why I'm doing it, but I think people are offended by it since it generally indicates a defensive posture as @Meg below indicated in her answer. I have included a picture below of me in this posture. In the picture, you will notice that the thumb is not clenched.