backbreaking
1. [ adjective ] characterized by toilsome effort to the point of exhaustion; especially physical effort
Synonyms: hard arduous punishing heavy laborious labourious gruelling toilsome grueling
Examples:

"worked their arduous way up the mining valley" "a grueling campaign" "hard labor" "heavy work" "heavy going" "spent many laborious hours on the project"

Related terms: effortful
Similar spelling:   back_breaker
  back_burner