A particle used for emphasis meaning 'even' (e.g., even A). It highlights that something applies without exception or is surprising.
子どもですらその簡単な問題を解ける。
Even a child can solve that simple problem.
彼のミスは上司ですら見逃さなかった。
Even his boss didn't overlook his mistake.
準備をしていない人ですらすぐに気づいた。
Even people who weren't prepared noticed immediately.
その古い本は研究者ですら読めない古語が多い。
That old book contains many archaic words that even researchers can't read.