Quoth Edict, たりとも means "(not) even; (not) any", and is probably used as such:
一問たりとも答えられなかった。(even one question, cannot answer)
Is たりとも made up of the individual portions たり ("such things as") and とも ("even if")?
What's the etymology of たりとも?
What are some other ways to analyze its construction?