Used in negative sentences to emphasize that not even a small amount or number exists. Follows a counter or quantity word (e.g., 一人たりとも, 一円たりとも).
一人たりとも遅刻を許さない。
I won't allow even one person to be late.
一円たりとも無駄にできない。
I can't waste even a single yen.
さえ means 'even' and can be used in both positive and negative sentences, whereas たりとも is only used in negative contexts with a counter.
Derived from the classical auxiliary verb たり (indicating completion or state) combined with the particle とも (even if). The modern usage is a fixed expression meaning 'even' in negative contexts.