I was trying to tell a language-exchange friend that I didn't know something with the somewhat express connotation that I should have (known).
The first thing that came to mind was しまう, which usually expresses unintended action that is done. However, this immediately became a problem, as I did not know how to use しまう with the negative て form. According to this topic, ないでしまう is acceptable.
Yet, this usage seems to be rather rare.
In short, if I were to say, "Oh, shoot, I didn't know that," what is the most natural and/or common way to express this? Is 知らなかった alone enough?