heterophemy

Century Dictionary and Cyclopedia

1. n. 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.

Wiktionary

1. n. The use of one word or phrase when another is meant

GNU Webster's 1913

1. n. 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.