hyposulphurous

from The Century Dictionary.

• Next in a series below sulphurous: used only in the following phrase.
• A totally distinct acid, H₂S₂O₃, now called thiosulphuric acid. See thiomlphuric.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Chem.) Pertaining to, or containing, sulphur, all, or a part, in a low state of oxidation.
• adjective [Obs.] An acid, H2SO2, obtained by the reduction of sulphurous acid. It is not obtained in the free state, but in an orange-yellow water solution, which is a strong reducing and bleaching agent. Called also hydrosulphurous acid.