The word militas is the present form of militar in the second person singular.
See the full militar conjugation

militar

¿Qué haces exactamente, militas?
What do you do? You campaign?
En una década como la de los 70 no había como eludirlo. Y claro, si estudias Ciencias Políticas y Administración Pública, pues militas en la izquierda.
In a decade like the 70s, there was no way to avoid it. And, of course, if you study Political Science and Public Policy, you join forces with the left.
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