Having the quality of flexibility without brittleness; yielding to force without breaking. The ligaments of animals and of India rubber are remarkably tough. Tough timber, like young ash, is the most proper for the shafts and springs of a carriage.

Firm; strong; not easily broken; able to endure hardship; as an animal of a tough frame.

Not easily separated; viscous; clammy; tenacious; ropy; as tough phlegm.

Stiff; not flexible .