To act cool means to appear relaxed, composed, and nonchalant in social situations. There are different synonyms that can be used to describe this behavior. One word that can be used is "calm", which means to be free from agitation or nervousness. Another synonym is "collected", which connotes someone who is composed and in control. Remaining "unperturbed" or "unruffled" are other synonyms that depict the act of remaining calm and not showing any signs of anxiety or stress. Ultimately, the ability to act cool is a desirable trait that can help anyone navigate social situations with ease and confidence.