The phrase "weak constitution" refers to a person's physical or mental state that is not strong or sturdy. It means someone has a fragile or easily compromised health or easily affected by problems. Full definition
Since my childhood I have a very weak constitution. (drlam.com)
These two methods are not without risks: the high intensity method is not recommended for people with weak constitutions because it will definitely push your muscles and your whole physical frame to their limits. (howtobuildmuscless.com)
Those with weak constitutions have higher propensities to develop Adrenal Fatigue even under normal daily living. (drlam.com)