"Dry" means close-miked, and recorded in a venue with higher sound-absorption properties than, say, a concert hall, which would confer reverb on the recording. Original soundtracks were typically recorded this way, especially from the 1930s through the '60s, on scoring stages covered in padding that hampered sound reflection.

Too bad Ed Nassour isn't here to expound on this, as he has more insight than I in these matters.