A wally is a silly, foolish person — someone who does something stupid, forgets something obvious, or makes themselves look ridiculous. It is British slang, and the important thing about it is how gentle it is. Calling someone a wally is much softer than calling them an idiot or a fool. Friends say it to each other, and people say it about themselves: "I felt a right wally standing there in the rain." It is about one silly moment, not about someone being stupid in general. It is often used with "right" or "proper" in front of it, which makes it stronger but no less friendly. Two things to know about how it sounds today. It is quite old-fashioned — it was very common in Britain in the 1970s and 80s, and younger people now use it much less, so it can sound like something your dad would say. And it is almost only British; Americans will not know it. Wally is also a man's name (short for Walter), so context tells you which one is meant.