We don't have synonyms for coalsack yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are coatrack, coal sack and cocksack.
°A member of a people of Eastern Europe and the adjacent parts of Asia, that eventually settled in parts of the Polish-Lithuanian Commonwealth, the Russian Empire and the Soviet Union, particularly in areas now comprising southern Russia and Ukraine.
°A member of a Cossack military unit (typically cavalry).
°A member of a people of Eastern Europe and the adjacent parts of Asia, that eventually settled in parts of the Polish-Lithuanian Commonwealth, the Russian Empire and the Soviet Union, particularly in areas now comprising southern Russia and Ukraine.
°A member of a Cossack military unit (typically cavalry).