To make someone laugh, we sometime rub his underarm with our fingers in a way that makes him feel restless and then he starts laughing. My question is: What is the specific vocabulary in the English language for the aforesaid rubbing?
At the risk of giving too general a term, I would suggest the word "tickle". This would refer to rubbing any part of the body to elicit laughter.
Definition 2 here: http://www.merriam-webster.com/dictionary/tickle