Probably related to Albanian bashkë "together, simultaneously", also used as a prefix akin to English co- (example: bashkë- + punoj "work" = "cooperation, collaboration").Source: Wiktionary (http://tinyurl.com/zs4sdxg).
It is possibly from Old Albanian or borrowed from a different language or possibly a different dialect because according to the Albanian dictionary unitet is the Albanian word for Unity.
https://en.wiktionary.org/wiki/bashkim