sharp as a razor

1. very sharp. (*Also: as ~.) The penknife is sharp as a razor. The carving knife will have to be as sharp as a razor to cut through this gristle.

2. and sharp as a tack very sharp-witted or intelligent. (*Also: as ~.) The old man's senile, but his wife is as sharp as a razor. Sue configure things out from even the slightest hint. She's as sharp as a tack.