Source : Webster's Revised Unabridged Dictionary (1913)

Soil pipe \Soil pipe\
   A pipe or drain for carrying off night soil.

Soil \Soil\, n. [OE. soile, F. sol, fr. L. solum bottom, soil;
   but the word has probably been influenced in form by soil a
   miry place. Cf. {Saloon}, {Soil} a miry place, {Sole} of the
   foot.]
   1. The upper stratum of the earth; the mold, or that compound
      substance which furnishes nutriment to plants, or which is
      particularly adapted to support and nourish them.

   2. Land; country.

            Must I thus leave thee, Paradise? thus leave Thee,
            native soil?                          --Milton.

   3. Dung; f[ae]ces; compost; manure; as, night soil.

            Improve land by dung and other sort of soils.
                                                  --Mortimer.

   {Soil pipe}, a pipe or drain for carrying off night soil.

Source : WordNet®

soil pipe
     n : drain that conveys liquid waste from toilets, etc.