An emphatic form of 'sae' meaning 'even' — used to indicate that something applies in an extreme or surprising case (e.g., 'even X').
子どもでさえも分かるように、もっと簡単に説明してください。
Please explain it more simply so that even a child can understand.
彼は忙しくて、昼ごはんを食べる時間さえもないと言っていた。
He said he was so busy that he didn't even have time to eat lunch.
そんなに人気のある映画なら、私でさえもチケットを買いたくなる。
If the movie is that popular, even I would want to buy a ticket.
彼のミスは小さいが、上司でさえも見逃さなかった。
His mistake was small, but even his boss didn't overlook it.