Hi Angliholic In both cases the meaning is basically "in very good condition". More specifically, in the case of the mammoth, that means the body has been well-preserved and undisturbed. In John's case, it could be fairly context-dependent, but it probably means he is phsically fit or quite healthy.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.