problematical, burned, thickened, enured, treated, problematic, hard, annealed, sunbaked, bad, tough, rugged, baked, ruffianly, case-hardened, burnt, hard-boiled, inured, sturdy, knotty, set, weather-beaten, elusive, baffling, calloused
strengthened (part of speech: verb)
- This old world's day is now passed, and I realize in that fact the reason for its unseemly behavior, first knocking its toughened crust so rudely against the earth and then coquetting in this manner with Mars. - "Daybreak: A Romance of an Old World", James Cowan.
- The nomads were trained and toughened by their weeks of fight for survival, but Mayfield's men were only weakened by their strained effort to keep the town alive. - "The Year When Stardust Fell", Raymond F. Jones.
- He soon had the knack of that: his muscles were toughened already. - "The Reign of Law A Tale of the Kentucky Hemp Fields", James Lane Allen.