Nicola also inquired about the terminology, and Dupuit's 1844 article cited 
by Manuela seems to make no reference to "deadweight loss" (or similar).  In 
1844 Dupuit was clearly working with the general notion and showed it as a 
triangle ("and the loss of utility becomes the triangle RTN;" see the 1996 
reprint in The Foundations of Public Finance, Vol. II, E. Elgar).  His many 
references to the concept were translated as "loss of utility," "loss to 
society," and "loss of utility to society."

The translation of Dupuit (1849), meanwhile, contains "dead loss," as in 
"dead loss for everybody" (p. 8 of the 1998 reprint in The Foundations of 
Regulatory Economics, Vol. I., E. Elgar).

Torsten Schmidt