The field label is stored in the model ir.model.fields. You can find a definite object by its name and related model. An example:

field_id = self.env["ir.model.fields"].sudo()._search([
    ("name", "=", "price_unit"), 
    ("model", "=", "sale.order.line")
], limit=1)
label = field_id and field_id.field_description or "Not defined"

Take into account that the name would be shown in the SuperUser language. In order to show it a different language you should pass the context with a required lang code.