Like "three-on-the-tree" for a 3spd column shift manual gearbox, or "four-on-the-floor" for a 4spd floor shift manual gearbox, "Two-in-the-Goo" refers to a 2spd Automatic transmission, most obviously a Chevrolet / G.M. Powerglide.
Two denoting the number of forward speeds, and "goo" describing the slushy, slippy nature of an automatic trans.
This old clunker came with a 283 with Two-in-the-Goo originally, but we quickly sorted that problem out with a 383/Muncie Four-on-the-Floor combo years ago and haven't looked back.