To choose out ( one); separate.
Singly.
Consisting of one only; individual; alone.
And so Mr. Penfold is at home, after all; and not to send me a single line. – Foul Play by Charles Reade Dion Boucicault
I've never told a single soul. – The Romantic by May Sinclair
Then to lose them both in a single day! – Cambridge Sketches by Frank Preston Stearns