To thicken, especially by boiling, evaporation, or condensation; condense.
To become viscous.
Thickened or dried by evaporation
(metaphorically) concentrated or accentuated
As for the Great Reform Bill itself, his gloom was inspissatedː "I cannot see what is to save Church, or property, or colonies, or union with Ireland, or eventually monarchy, if the Reform Bill passes." (Ferdinand Mount, Times Literary Supplement 21 August 2015 page 4, referring to the Duke of Wellington)