On Samuelson's deadweight loss--following John Womack's sighting in the
9th edition:

Using the lazy way out of just looking in the index,
the term deadweight loss also appears in the index of the
8th edition of Samuelson (1970) but NOT in the
index of the 7th edition (1967) NOR in the index of any earlier
edition (i.e. 1 through 6). Whether a careful reading of the text in
editions prior to the 8th would reveal the term in the text though not in
the index I will for others to pursue.

The term  does appear in the index of the 15th edition of
Samuelson and Nordhaus (1995).

David Mitch