Rashi on Exodus 10:26/רש"י על שמות י׳:כ״ו

1פרסה is the hoof of the foot; in old French plante.

2לא נדע מה נעבד means WE DO NOT KNOW WITH WHAT WE MUST SERVE we do not know how heavy will be the service: perhaps He will ask more than we have in our possession.