Rashi on Exodus
מתניכם חגרים YOUR LOINS GIRDED — i. e. prepared for the journey (Mekhilta d'Rabbi Yishmael 12:11).
Sforno on Exodus
מתניכם חגורים, ready to commence marching. We find a similar expression in connection with the prophet Elijah in Kings I 18,46 where he is described as וישנס מתניו, “he bound up his loins.” The expression reflects absolute confidence in G’d’s assistance of the undertaking one is about to embark on. This is displayed while the prisoner is still inside the walls of his jail, the door not having opened yet to let him out.
Rashbam on Exodus
פסח הוא לה', to insure that G’d’s angel killing the firstborn will bypass the houses with the sign of the blood on their door-frames.