A particle meaning 'even' used for emphasis (similar to さえ). Often appears with negative or surprising statements to stress that even the listed item applies.
彼は助けを求めることすらしなかった。
He didn't even ask for help.
その問題は小学生ですら解ける。
Even elementary-school students can solve that problem.
お金がないどころか食べる物すら買えない。
Not only is there no money, he can't even buy food.