Yes, "utilit??? perdue."

I would still be interested in knowing what Dupuit's exact words were in his
1849 "On Tolls and Transport Charges" ("Des p???ages") which were rendered as
"dead loss" by the translator, Elizabeth Henderson.  The translation was
first published in 1962, after the term "deadweight loss" had become fairly
common; this might have influenced her choice of words.

Does anyone have convenient access to the original text in French?  In case
someone does, and to make the expression easier to locate, it should be
appearing near the end of a paragraph somewhere on pp. 215-217 of the
original page numbering from 207 to 248.  In the translation, the full
sentence reads "Whatever the advantages of this distribution, they are not
comparable with the 14,900 francs of additional utility which are gained by
public operation and which are a dead loss for everybody in the case of a
private company."

It may be of interest that Jevons seems to have used "dead loss" in 1884: 
"... the very scarcity of gold is its recommendation ... in itself gold 
digging has ever seemed to me almost a dead loss of labor as regards the 
world in general" (p. 104 of "Investigations in Currency and Finance," as 
quoted by Fisher on p. 87 of "Appreciation and Interest," 1896).

Torsten Schmidt