“Être bête comme ses pieds” is one of my all time favorite French expressions. The literal translation is “to be stupid like his (or her) feet”. The idiom has lots of English equivalents including “to be dumb as a bag of nails”, “dumb as a rock”, “dumb as a doorknob”, “thinkheaded” and “so stupid”, among others.
Être bête comme ses pieds

A French-to-French definition is simply être très bête or très stupide (to be very stupid).
According to Expressio.fr, this expression dates from the nineteenth century, where the feet were said to be the body part the furthest from the head or “seat of thoughts”, and thus associated with total stupidness or stupidity.
Here’s an example sentences:
Le président avait plusieurs mois pour réponder à la crise sanitaire et il ne fait que nier la situation. Il est bête comme ses pieds!
The president has had several months to respond to the health crisis and all he’s done is deny the situation. He’s as dumb as bag of nails.
Le maire de cette ville est bête comme ses pieds. Ça fait quatre ans qu’il a son poste et il n’a rien fait!
The mayor of this city is dumb as rocks. He’s been in his position for four years and he hasn’t done anything!
Note: There’s another common French expression which uses bête: “bête comme chou“. The literal translation here is “stupid” as cabbage” but the meaning is “easy as pie”, “dead easy” or “dead simple”.
Conclusion
Congratulations! You now knwo how to use the expression, bête comme ses pieds. Now to check out our post covering another related expression “casser les pieds“ (to annoy).