J'ai la flemme. Does this sound like something you would say if you have the beginnings of a nasty cold? Maybe, but this phrase actually means something quite different: it means I'm in no humour to do something because I can’t be bothered, and I'd rather be doing a less energy-consuming activity instead. It could also translate as the milder I don’t feel like it, as long as a strong sense of apathy is still taken into account.
J'ai la flemme (de faire quelque chose) is an informal expression that adds a bit of drama to the situation at hand when compared to the (slightly) more formal je n'ai pas envie - I don't feel like. It takes its fullest expression if uttered with a sigh. A French sigh; slow and consequential. Even better if your face shows mild signs of existential pain.
You can also say Je suis un(e) flemmard(e) if you see yourself in a perpetual existential crisis leading to chronic apathy - or just want to call yourself a lazy person.
The French flemme and the English “phlegm” both come from the Latin word phlegma. In the Middle Ages, phlegm referred to one of the four humors: blood, yellow bile, black bile and phlegm. Phlegmatic people were supposed to be apathetic and that's the meaning of the phrase avoir la flemme today. The word la flemme alone translates as laziness and can be used as an informal synonym of la paresse.
Je ne veux pas faire le ménage. J’ai la flemme.
I don’t want to clean the house. I can’t be bothered.
J’ai la flemme de travailler aujourd’hui.
I don’t feel like working today.