It’s a slang term for pushing a kid on a swing and then running forward underneath it before the kid can swing back down and bonk you on the head with his feet. I was told about it by a guy who said that, growing up in NE Ohio in the 1970s, it was a common term. I grew up in a different town but in the same region and time, and I’d never heard it before. Had you?
Tell me your age and where you grew up, please. Thanks!
NE Ohio in the 80s - never heard of it.
SE Minnesota in the 90s. I’m very familiar with the term.
NE Ohio in the 80’s also. Never heard of it.
NE Illinois
I’m familiar with the term.
New York City, born in '80. This is my first time hearing of either the term or the practice.
Michigan in the eighties, and yes, I do have a faint recollection of it. Only we never used “underdogging”, quite, we used to say, “Give me an underdog! Underdog!”
Massachusetts in 80s. We did that, but it never had a name.
Michigan in the '60’s. Sure, If someone was going to push you, getting an underdog was hte way to go.
StG
I never heard of this. If someone had mentioned “underdogging” to me then I would have thought it was a reference to the cartoon.
For the OP’s question, I am approaching 39 years of age. I’ve lived in Idaho for all my life.
Central NJ, mid-late 70’s - Yes! I loved the Underdog. I always thought it was related to the cartoon Underdog, who I also loved.
Western NY, late 70s, yeah, we did the Underdog. (Except those metal cage swings which seem to be unique to that area and time, where you sat in a metal cage and rhythmically pulled handlebars to swing. Doing the underdog on those would be dangerous due to the heft and herky jerky motion of the swing.)
Huh…same location and time frame as meow meow, but this is the first time I’ve heard the term (outside of the cartoon, of course) or the action.
Never heard of it. Southern Ontario, 1970s.
Mostly grew up in Michigan, did some growing up in Ontario, Canada and I did hear the term in both regions. Grew up in the 1970s. Totally forgot about it until I read the OP though.
I don’t think I heard “underdogging” it was always “to give an underdog”.
I also grew up in NE Ohio in the '70s and this thread is the first time I’ve heard the term.
NE Ohio, '80’s – yes, it was a common term.
–Cliffy
Michigan in the 70s. I definitely knew the term and engaged in plenty of underdogging, at least until the accident. 
Tsk! You really missed out on some sweet, sweet backyard swing action…I don’t consider some towns in NJ to really be part of NJ, maybe you are from one of them?
Detroit in the early 90s. Yes, it was a common term.