Early 17th century; earliest use found in Richard Knolles (d. 1610), historian and translator. From co- + ally, originally after Middle French coallié.
Definition of co-ally in US English:
co-ally
nounˌkəʊˈalʌɪ
A fellow member of a coalition or alliance.
Origin
Early 17th century; earliest use found in Richard Knolles (d. 1610), historian and translator. From co- + ally, originally after Middle French coallié.