The shoemaker's son always goes barefoot

En casa del herrero cuchillo de palo, en casa del herrero asadón de palo. - it's when someone who can do the job they don't do it because they're lazy.

Example:

The doctor's son is very sick, yeah the shoemaker's son always goes barefoot - El hijo del doctor está muy enfermo, si, en casa del herrero asadón de palo.