Boca stole a flag (trapo) to River, from there the origin of the song, they will never see the flag.
Lyrics
That joy to joy, Ole ole ola! Let Boca yet! these to win, You ran in Mar del Plata, do not forget that, Cloth Di Carlo, You'll never see, La Boca is joy, La Boca is Carnival We will run a River in the Monumental... (Repeated...)