heterophemy

from The Century Dictionary.

• noun The saying of one thing when another is meant; specifically, a disordered or morbid mental condition which leads to the saying or writing of one thing when another is meant; physical incapacity to express one's ideas in language conveying a correct impression. When heterophemy becomes a pronounced disease it is known as aphasia. Also heterophemism.

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

• noun The unconscious saying, in speech or in writing, of that which one does not intend to say; -- frequently the very reverse of the thought which is present to consciousness.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The use of one word or phrase when another is meant