It is a . . . Ultra-Castilianization? of the French coéquipier ("companion of the same team") that is used a lot in Mexico and that has a previous version in "coequiper", widely used in Argentina although it does not fall into the category of lunfardo. In truth and at most it would be barbarism, since it does not exist in Spanish, where phrases such as its translation "teammate" are preferred.
It means partner, teammate, person who belongs to the same team. Colleague, collaborator, companion, gregarious. It is used mostly in sports slang.
The signicado is to the free will of the ignorant who uses it.
In cycling, the coequipero is the cyclist who escort a teammate when this is leading, supporting and encouraging him at all times. The coequipero can eventually take the lead in the race, where his partner get tired, for example.