In the GameEdit
Akira initially says his knife is military issue; he doesn’t remember when he first obtained the knife, but Akira assumes he recieved it during his training as a child soldier. On Nano’s route, he learns it was a gift from Nano. Nano says the meaning of the inscription is “a wish.” Nano didn't promise they would meet again, but he symbolically gave Akira the last of his hope.
In the Anime Edit
In episode 10, it is shown during a flashback sequence that Akira met Nano in the past. It is revealed that the knife Akira uses is actually a gift from Nano, as a young Akira promised to meet Nano again one day, after Nano declined to stay with him on their first meeting.