Technical term, stands for "done fell out." An appropriate use for this phrase is anytime somebody "done fells out." For example "Timothy was drinking his normal daily pint of whiskey when he done fell out" (actual quote from today in hospital).
Other appropriate uses are when you have done something that you don't want your doctor to know about (i.e., some type of "street pharmaceutical). Often if DFO phrasing is used, the story doesn't quite fit. For example "I was just walking in my back yard when I done fell out." The real story probably involves ingestion of antifreeze or similar chemical not meant for human ingestion in search of a high.
Another tip: if you have a DFO-er, be highly suspicious of cocaine use. This is especially true if said DFO-er is from Augusta, GA. Apparently Augusta didn't get the memo about crack falling out of vogue and is so stuck in the mid-1990's. So, if you suspect cocaine use and they are denying it or saying it has been years, then ask again, telling them it is really important. They will come back saying they last used a couple months ago. Again, stress the importance of how imperative it is to know exactly the last time they used it. This is where things will get fuzzy and they will pretend like they are thinking and will say two weeks ago. At this point, you tell them it is extremely important and that the only reason you are repeatedly asking them this is because they need a life saving medication, but if they have done crack recently it will kill them. At this point their memory will come to and they will admit to doing the crack just prior to DFOing. I couldn't make this stuff up.
