have the constitution of an ox
To possess an unusually robust amount of strength, determination, and stamina, so as to be able to work extremely hard and/or overcome hardships or limiting factors (e.g., sickness, fatigue, alcohol, drugs, etc.). John works his farm single-handedly every day, from sunup to sundown; he must have the constitution of an ox! Mary has the constitution of an ox—she's had more drinks than any of us, and she still seems completely sober. Janice was bedridden with the flu over the weekend, but she must have the constitution of an ox because she was right back in the office first thing Monday morning.