Why do I need to know gamin?
Because if someone calls you a gamin it can mean two different things.
What does it mean?
Firstly, gamin refers to a 'child'. However the expression is quite familiar so a better translation is probably 'kid'.
Un gamin means a young boy and une gamine means a young girl.
Dis-donc, il est mignon ton gamin ! – What a cute kid you have there!
Arrêtes de te comporter comme un gamin ! – Stop acting like a child!
Use it like this
Cette gamine est tellement intelligente. – That girl is so intelligent.
Je n'aime pas les gamins. – I don't like kids.
Il a envie des gamins depuis des années. – He's been wanting kids for years.
Profites, tu ne sais pas combien la vie change dès que tu as des gamins. – Enjoy, you have no idea how much life changes as soon as you have kids.
Synonyms
Enfant – child
Gosse – small child
Bambin – small child
Môme – small child
Surely the use of ‘gamin(e)’ that will be known to most readers here is that covered by the wikipedia article here: https://en.wikipedia.org/wiki/Gamine. Actresses such as Audrey Hepburn, Jean Seberg, Audrey Tatou and Anna Karina.