The word chupaban is the imperfect form of chupar in the third person plural.
See the full chupar conjugation

chupar

Las obreras chupaban los pinceles impregnados en radio repetidas veces, completamente inconscientes de los riesgos para la salud.
The workers would lick their paintbrushes soaked with radium time after time, completely oblivious to the health risks.
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