rellenar
/reʝeˈnaɾ/ · B2 · verb
to fill
📝 re- (again) + llenar (to fill, from lleno = full) = to refill
Llenar comes from lleno, meaning full, which comes from Latin plenus, the root of English plenty and replenish. Adding re- gives refill, and from there it also means filling in a form: rellenar un formulario.