'Boxer's attitude' is seen in person dying of the following condition:

Pugilistic Attitude (boxing, fencing or defence attitude): The posture of a body which has been exposed to great heat is often characteristic. The legs are flexed at the hips and knees, the arms are flexed at elbows and wrists and held out in front of the body, head slightly extended, all fingers are hooked like claws. Contraction of paraspinal muscles often causes a marked opisthotonus, in an attitude commonly adopted by boxers. This stiffening is due to the coagulation of proteins of the muscles and dehydration which cause contraction. Ref: Essentials of Forensic Medicine and Toxicology by KS Narayan Reddy, 27th edition, Page 286.