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 #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!!
You get the occasional normal person who says it.
Or if people aren't listening to you.. or ignoring you you can shout "DINGYED!"
Sean: Good, Paul is a fanny :\
Claire: So guys.. Wanna go see a movie later?
Group of Claire's friends: *Ignores* *Talks about pointless things*
Claire: Ahem..? *Ignored* :o DINGYED! :/
Group of Friends: Oh.. Sorry Claire..
"Dingy is the opposite of clean, love. It's dull, grayish, faded, grimy, grubby, mucky, and just dreadfully dirty"
2. (Thinks WTF???)
1. Here you go... (gives shirt back). Cheerio!