Hot body, cold body, working body diagram or view of the Carnot engine.
In thermodynamics, a hot body is the body of elevated temperature, in the structure of a heat engine, as compared to another, called the cold body, in which it is lower. In equation form:

TA (hot body) > TB (cold body)

The hot body is typically referred to as the furnace or boiler, i.e. burning coal or fire used to heat a tank of water, maintaining the boiling water at a constant temperature TA, which is thus serves as a heat source, as contrasted with the heat sink (the condenser or stream of cool water), readily able to give up heat to the working body of expanding and contracting substance (typically water) inside of the piston and cylinder.

History
The definition of the hot body was defined by French physicist Sadi Carnot in 1824 in his description of the production of motive power in steam engines or heat engines, in which he utilized the logic of the cycle or Carnot cycle. To quote:

“The production of motive power [work] in the steam engine is due the transportation of caloric [heat] from a warm body to a cold body, i.e. to its re-establishment of equilibrium—an equilibrium considered as destroyed by any cause whatever, by chemical action, such as combustion, or by any other.”

This terminology was taken up French engineer Emile Clapeyron in 1834 who graphically described the Carnot cycle utilizing the hot body / cold body terminology. [2]

Human thermodynamics
In studies of human thermodynamic systems, the location of the hot body is an intricate subject of study. The basic model of any generic social system is that the sun acts as the hot body. In studies of small number human molecule interactions, a hot body can be distinguished by a facet of perceptual physical or mental beauty. It is well documented, for example, that a supermodel walking through a crowd of people will cause of volume expansion around themselves, according to the rule that they will be allotted more personal space. In these types of studies, the determination of where the three different bodies (hot, cold, and working) are becomes a very complicated subject. [3]

The lyrics to the popular 1999 R&B song "Hot Boyz" by Missy Elliott gives an idea of the difficulties involved in trying to determine the three different bodies of the thermodynamic system. [4] It is clear that the guy (a) treating women good, (b) carrying a glock, (c) living by himself, (d) driving an expensive car, (e) using a platinum visa, (f) with friends who are hot, etc., is the "hot body" of the Carnot engine model, but the determination of the cold body and the working body becomes very elusive.

References
1. Carnot, Sadi. (1824). “Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power (excerpt)” Paris: Chez Bachelier, Libraire, Quai Des Augustins, No. 55.
2. Clapeyron, Emile. (1834). “Memoir on the Motive Power of Heat”, Journal de l’Ecole Polytechnique. XIV, 153 (and Poggendorff's Annalender Physick, LIX, [1843] 446, 566).
3. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview), (Google books). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two), (preview), (Google books). Morrisville, NC: LuLu.
4. Hot Boyz (lyrics) – AZLyrics.com.