(1):

(superl.) Not easily broken; able to endure hardship; firm; strong; as, tough sinews.

(2):

(superl.) Not easily separated; viscous; clammy; tenacious; as, tough phlegm.

(3):

(superl.) Having the quality of flexibility without brittleness; yielding to force without breaking; capable of resisting great strain; as, the ligaments of animals are remarkably tough.

(4):

(superl.) Stiff; rigid; not flexible; stubborn; as, a tough bow.

(5):

(superl.) Severe; violent; as, a tough storm.