What does this gesture mean? I've seen it three times. I care! What kind of dirty joke is it?

The thumb is sandwiched between the middle finger and the index finger, and there is a single middle finger, which is almost the same meaning. This is a gesture that imitates the male genitals and is used to swear. Because it is vivid, it means ML. Don't interpret it as a verb. More often, it is used as an modal particle, which is swearing words.