Pourquoi a-t-on la ''flemme'' ?

Le mot flemme vient de l'italien, flemma, qui signifie fatigue, lenteur.
On a la flemme lorsque l'on manque d'enthousiasme pour réaliser une action, un travail.
C'est un mot familier.

Complement internaute :
English: - [Voiceover] The French word 'lent' comes from the Latin word 'flamma' which means flame. Lent is when you feel like you're just barely holding on. You can be lent with someone else, you can be lent with a situation. When you're lent, it can be a physical thing, you're physically lent, your muscles are aching, or you can be lent mentally, you're mentally lent, you just don't feel like you're doing what you're supposed to be doing. When you're lent, you feel like you're barely hanging on. Lent comes from the French word 'flamma', which means flame, so when you're lent, you can feel like you're just barely holding on to the flame. You might be in a situation where you're in the dark and you're just barely holding on, and you might feel like you're in a situation where you just need a little bit of extra energy to just get yourself moving.