Anybody know how to say "underdog" like when you push someone on the swing and run all the way underneath them, that's doing an "underdog.

Thanks. ;D

Tal vez puede ser "empujar muy recio a alguien en un columpio"

Tal vez un hablante nativo nos puede ayudar tambien.

I didn't know this term. For me, an underdog was a "subestimado", "tapado" or something like that, somebody not expected to win and "desvalido","desamparado" for somebody with no resources/oppressed.
We say "columpiar" when you are in the swing moving alone, and "empujar al del columpio" "empujar el columpio" or something like that. "dar impulso" or "impulsar" is also correct, but not usuall (to give impulse), but I can't come for a better term.
And "columpiar" can also be "equivocarse" "El se columpió cuando pensó que el coche era suyo" is like saying "El se equivocó cunado pensó que el coche era suyo" (He was wrong thinking the car was his"