en casa de herrero, cuchillo de palo
/en ˈkasa de eˈrɛ.ɾo ˈku.tʃi.ʎo de ˈpa.lo/idiom★Intermediate◆idiom
informal
A person who has a lot of knowledge or skill in a particular area may not be very good at applying it to their own life or situation.
El hijo del herrero es un cuchillo de palo.
The blacksmith's son is poorly dressed.
📖Word Origin
This idiom comes from the idea that a blacksmith (herrero) has the skills and tools to make good knives (cuchillos), but his own child (or family member) may not benefit from his expertise, being poorly dressed (cuchillo de palo) or lacking the advantages that one would expect.
📝Usage Notes
This expression is often used to describe situations where someone with great expertise or resources fails to apply them effectively to their own situation or to benefit those close to them.
Diccionario Español-Inglés