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).
I won't allow even one person to be late.
I can't waste even a single yen.
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.