from Wiktionary, Creative Commons Attribution/Share-Alike License
- v. To deprive of synonymous character; to cause no longer to be synonyms.
from the GNU version of the Collaborative International Dictionary of English
- transitive v. To deprive of synonymous character; to discriminate in use; -- applied to words which have been employed as synonyms.
from The Century Dictionary and Cyclopedia
- To deprive of synonymous character, as words of similar meaning; differentiate in signification; discriminate (synonymous words or phrases). Also spelled desynonymise.
Sorry, no etymologies found.
It is not, I own, easy to conceive a more apposite translation of the Greek phantasia than the Latin imaginatio; but it is equally true that in all societies there exists an instinct of growth, a certain collective, unconscious good sense working progressively to desynonymize  those words originally of the same meaning, which the conflux of dialects supplied to the more homogeneous languages, as the Greek and German: and which the same cause, joined with accidents of translation from original works of different countries, occasion in mixed languages like our own.