Avoir un poil dans la main literally means "to have a hair in the hand," which doesn't make a lot of sense at first. The expression is used to convey that someone is "extremely lazy," "bone-idle," or, quite simply, "a lazybones."
Though it's unclear as to where the expression originated, it's often attributed to the late 1800s. The reason this expression refers to laziness likely has something to do with manual labor, which was standard at the time. It also requires an exercise in visualization to get the full picture: hold one hand palm facing up, and make the gesture of a hair growing in it with your other hand 😵
Avoir un poil dans la main and its accompanying gesture offer a very precise image of a hand that is so idle there's actually a hair growing out of it. This is assumed to be because a hand that does not work is not subject to the friction that inhibits hair growth on the palm. The biological facts of how rare hair growth is in the palm calls this into question. And yet, we must also remember that this is something French peasants centuries ago may not have understood clearly 🥴
Using the Expression in Context
How would you use such an odd expression in conversation? It's actually quite easy and can simply be added to express the extent of the laziness you're trying to explain.
Il a un poil dans la main, c'est pour ça qu'il ne réussit pas.
He is lazy, that’s why he doesn’t succeed.
To express even greater laziness, you can take it a step further by increasing the size of the hair. Both of these are common variations on the expression:
Il un pieu dans la main - He has a stake in his hand
Ce n'est plus un poil (qu'il a dans la main), c'est un bambou ! It's no longer a hair (that he has in his hand), it's a bamboo stick!