Erg

The energy unit of the centimetre-gram-second system, being the work done by one dyne acting through one centimetre.

Before the metre-kilogram-second system prevailed, physicists worked in centimetres and grams, and the erg was the energy unit that followed from those choices. It is minute: a housefly in flight expends a few hundred thousand of them each second, and a single joule contains ten million. The name comes from the Greek ergon, work, which also gives us energy and ergonomics. The erg survives in some astrophysics, where the enormous numbers involved make a very small base unit surprisingly convenient.