If your friend is acting cool, unconcerned or in an indifferent manner, call him nonchalant — like when he saunters by a group of whispering, giggling girls and just nods and says, "Hey."
If you act nonchalant, you are literally acting cool, as nonchalant traces back to non- "not" and Latin calēre "to be warm." Isn't that cool? Sometimes, a nonchalant person acts indifferent or uninterested, but really cares very much. If you give a girl a nonchalant smile, you definitely want her to notice you! Even though it begins with non, nonchalant has no positive form — chalant is not a word.