Literal Translation
To have the potato/the fry
Actual Meaning
To be full of beans
Etymology
It is because of its rather round shape that in early 20th century slang, the potato, or patata, was likened to the head, as the pear had been before.
According to some sources, although this is not certain, this expression is used because someone who is in good shape has a good “patate” and, via a form such as “il a une sacrée patate” (he has a hell of a patate), we arrived at the final expression “il/elle a la patate” (he/she has the patate).
As for ‘frite’ (chip), one of the derivatives of this expression, it follows quite logically, but later, in the 1970s. It was in fact from 1950 onwards that “frite”, like ‘patate’, came to refer to the head, as a simple joke since, according to the latest news, chips are made from potatoes. Then, the influence of this expression did the rest.