In an article in the last issue of this journal, Anil uncovered a number of examples of pseudoantonyms in the realm of English negatory prefixes. A pseudoantonym consists of an affixed word and the corresponding unaffixed base which one would expect to be antonyms on the basis of the meaning of the affix, but which are instead synonyms (or near synonyms). For example, the prefix DE- has a primary meaning "to do the opposite of" in examples such as ACTIVATE ~ DEACTIVATE, but not in FEND DEFEND. Since the latter two words can be synonymous, they are pseudoantonyms.
Hall, T. A.
"More on Pseudoantonyms,"
Word Ways: Vol. 44
, Article 24.
Available at: http://digitalcommons.butler.edu/wordways/vol44/iss2/24