Hi dear Love-to-learn
As usual you're right!I just looked this word up in Wikipeida encyclopedia and the result was exactly wt u said!
Here is the explanation mentioned [ برای مشاهده لینک ، با نام کاربری خود وارد شوید یا ثبت نام کنید ] :l
Underdog, the term describing a push on a swing where the person pushing runs underneath the person on the swing while pushing, resulting in a very high swing. The pusher often recites "Underdog, Underdog, Underdog", while pushing. Very popular with primary school children. The term and activity were created by Terrance Sweeney of Toronto, Canada in the mid-1970s.l
Of course it wasn't mentioned in any dictionary!l
Good Luckl