v. To deprive of synonymous character; to cause no longer to be synonyms.
the GNU version of the Collaborative International Dictionary of English
v. To deprive of synonymous character; to discriminate in use; -- applied to words which have been employed as synonyms.
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.