37 °C is the average of a measurement series from 1868.
The famous figure comes from Leipzig physician Carl Reinhold August Wunderlich, who published it in 1868. Modern measurement series find a mean closer to 36.4 °C, and an analysis published in 2020 concluded that average human body temperature has fallen since the 1850s.
Useful: A reference number is always a snapshot of the population it was measured in.
Source: en.wikipedia.org