class act is a noun.you use it together.It means showing excellence in action,or elegent act or behaviour.OR cool move,perfect.
Class in here meaning - high quality,perfection,noble,excellent etc.- it came from classis (I think) in the ancent Roman time.Roman distinguished her citizens into different classes.Classis = first class.
act - behaviour or activity or action
However,if someone said you are in a different class --> this can mean good or bad.Thank u! "class act" and "act" are all nouns.However,"class" here is an adjective or noun??class act is a informal noun. you use these 2 words together. If you have to individualise - class is an adjective and act is the true noun.thx much! Not to use separately, I mean, in this set phrase or word combination of "class act", does "class" as the adjective modify the noun "act" ?class can also be a noun too!