1. Characterized by effort to the point of exhaustion; especially physical effort
   "a gruelling campaign"
   - arduous, backbreaking, grueling [US], hard, heavy, laborious, operose, punishing, toilsome

See also: effortful

• subalpine fir
• subalpine larch
• subaltern
• subaquatic
• subaqueous
• subarachnoid space
• subarctic
• subartesian
• subatomic
• subatomic particle
• subaudition
• subbase
• subbing
• subclass
• subclass Acnidosporidia
• subclass Actinopoda
• subclass Alismatidae
• subclass Amphineura
• subclass Anapsida
• subclass Archaeornithes
• subclass Archosauria
• subclass Arecidae
• subclass Asteridae
• subclass Branchiopoda
• subclass Caryophyllidae