The Spanish-language wikipedia offers this on the meaning of the song:
Carlos Gardel le preguntó en público a Enrique Santos Discépolo que qué había querido hacer con esta canción. Enrique le respondió que quiso hacer una canción de soledad y desesperanza; trata de un hombre que ha vivido la bella esperanza de la fraternidad durante 40 años y de pronto, a esa edad, se desayuna con que los hombres son unas fieras. La canción lo expresa con cosas amargas.
My translation:
Carlos Gardel publicly asked Enrique Santos Descépolo [the writer of the song] what he was trying to do with this song. Enrique responded that he wanted to make a song of loneliness and hopelessness; he trying to show a man who had lived a life of beautiful hope and brotherhood for 40 years and suddenly, at this age, discovered for the first time that men were beasts. The song expresses this with bitterness.
Of course, that doesn't really explain the title. But the same article also has this note on censorship of the song:
A partir de 1943 en el marco de una campaña iniciada por el gobierno militar que obligó a suprimir el lenguaje lunfardo, como así también cualquier referencia a la embriaguez o expresiones que en forma arbitraria eran consideradas inmorales o negativas para el idioma o para el país obligó a reformar algunos títulos y letras para permitir su difusión radiofónica y así Yira, yira pasó a llamarse Camina, camina.
My translation:
At the beginning of 1943 in the case of a campaign initiated by the military government that compelled the supression of “thieves‘ cant“¹, as well as any any reference to drunkenness or expressions which in arbitrary form were considered immoral or negative for the language or for the country and required the reform of some titles or lyrics to permit their radio broadcast and so Yira, yira became Camina, camina [walk on, walk on].
The footnote on my translation offers some interesting background on the language, but the bottom line is that yira, yira was considered inappropriate language, and while that might lead us to interpret it as prostitute (which is the predominant survival of the term and the only definition offered by the Diccionario de la lengua española (this is roughly the Spanish-language equivalent to the English language's OED)). I think the key aspect was the consideration that lenguaje lunfardo was considered “negative for the language” and given the revised name of “Camina, camina“ suggests that it was meant as a verb and we should go with the meaning of “be on the game“ (which, incidentally, the Collins Dictionary defines as “If a man or woman is on the game, he or she is working as a prostitute.“). I think though, that the meaning is meant more metaphorically than literally and indicates here that if everyone is awful, as is the world, then you might as well not bother to be good. It is, after all, a song of hopelessness, loneliness and bitterness.
- The original Spanish term, lenguaje lunfardo, literally means thief language and the wikipedia article links to the page lunfardo talking about how the term was part of a jargon common to criminals in Argentina and Uruguay and has its origins with late 19th century immigrants from Italy mixed with other immigrant and indigenous populations' languages. It became spread throughout all classes of Argentine society courtesy of its usage in tango lyrics. All of which makes this footnote perhaps the most important part of my answer.
