intrepid
Meaning: brave in the face of danger
Usage: After scaling a live volcano prior to its eruption, the explorer was praised for his intrepid attitude.
hardy
Meaning: robust, capable of surviving through adverse conditions
Usage: I too would have expected the plants to be dead by mid-November, but apparently they’re very hardy.
fortitude
Meaning: strength, guts
Usage: Achilles’ fortitude in battle is legendary.
indefatigable
Meaning: tireless; unable to be fatigued
Usage: Even after traveling 62 miles, the indefatigable runner kept on moving.