Dingy is a word that can be used as an adgective to describe someone who is forgetful and makes silly choices at times. It's similar to ditzy, only usually implies it as a less intensified. Often times it refers to someone who has less than average common sense and/or simply doesn't think about what they are doing, as well as frequently forgetting about what they are doing or what they have planned.
It is used both to refer to someone who IS dingy and to someone who is BEING dingy. This means that it can mean that a person is always dingy or often is, however can also be used to refer to when someone makes a dumb choice or forgets something obvious. Everyone does something dingy at some point or another. That doesn't make them dingy, but their silly choice or forgetting something big is.
In the examples below, Susan is dingy because she makes a habit out of forgetting to turn the oven off, and probably forgets similar things. Jon and Kristen both acted dingy, but are not necesarily dingy themselves.
Many slang terms mean different things in different regions. In fact, they often mean very different things, hardly resembling each others' definition despite being the same word. This is the definition most commonly found in California.
This term is interchangeable with the slang usage for "ding-dong" when referring to someone who IS dingy. Someone who is dingy is also a ding-dong.
Gossiper #1: "Susan left the oven on last night."
Gossiper #2: "Again?"
Gossiper #1: "Yeah, she's a bit dingy."
Jon was going to meet his girlfriend for a date at a restaurant, but instead of going to the restaurant Jon accidently went to the grocery store. He had to turn around and arrived late at the date because of it. That could be described as being dingy.
Kristen heated her food in a metal bowl in the microwave, breaking the microwave and shorting the power temporarily becuase the bowl was metal. This would be very dingy!!