Why do I need to know pousser mémé dans les orties ?

Because you will often meet people who need to hear that.

What does it mean ?

Pousser mémé dans les orties can be translated into English as ‘to push granny in the nettles’.

Mémé is a familiar term for granny, although these days mamie is a more common alternative to the formal grand-mère.

The expression is always used in its negative form, which gives faut pas pousser mémé dans les orties (don't push granny in the nettles).

It was originally used simply as faut pas pousser, meaning ‘do not exaggerate’, and the rest of the expression was added in order to make it more colourful and emphasise the meaning.

It's now more commonly used to mean 'don't push it' or 'don't go too far' for someone whose behaviour is teetering on the brink of crossing the line. 

For example, if your sister borrowed your stuff without telling you and also stole your pocket money, you could have said faut pas pousser mémé dans les orties.

Use it like this

On ne s’est pas parlé pendant des mois, et il m’a appelée pour qu’on se remette ensemble.Faut pas pousser mémé dans les orties – We haven’t talked in months and he just called to ask me to take him back, that’s pushing it!

Je me suis occupé de son déménagement et maintenant elle veut que j’aille faire ses courses, faut pas pousser mémé dans les orties ! – I took care of her house move and now she wants me to go grocery shopping for her, she’s going a bit far!

Synonyms

C’est fort de café ! – That’s pushing it!

Pousser le bouchon – To push the boundaries

Abuser de la gentillesse de quelqu’un – To take advantage of somebody’s kindness