Why do I need to know mamie et papi?

Because it's adorable! But also useful.

What does it mean?

They might sound like affectionate alternatives to 'mum and dad', but mamie et papi is actually French for ‘grandma and grandpa’.

Mamie is a colloquial version of grand-mère (grandmother) and papi of grand-père (grandfather). (Papi can also be spelled papy too.)

Mamie et papi are probably the most common ways to address grandparents in France. There are other affectionate nicknames such as mémé and pépé, but they're a bit rarer, and then there are those grandparents who prefer the less informal options.

It really depends on the family. Some French families have a more old fashioned take on things, and some grandkids even address their grandparents with vous (the formal version of 'you') instead of tu. Although such formalities are becoming increasingly rare.

Use it like this

Dans mon village, les papis jouent à la pétanque avant de commencer l'apéro. – In my village, all the old men play pétangue (bowls) before the aperitif. 

Quand est-ce que mamie et papi arrivent ? – When are granny and granddad coming?

Tu ne la connaissais pas, mais ta mamie était une vraie rebelle lorsqu’elle était ado. – You didn’t know her then, but your grandma was a real rebel during her teens.

Fais gaffe, papi triche toujours quand on joue aux cartes. – Careful, grandpa always cheats when we play cards.

Synonyms

Mémé et pépé – granny and grandpa

Mémère et pépère – granny and grandpa (however mémère and pépère is apparently common in Canada than in France and can even have a pejorative undertone here).