Thank you Tom for lethologica, which means a state of not remembering the word for something, or forgetfulness of words. As usual the Wikipedia tells all. It suggests that "According to the American Psychiatry Association, '9 out of 10 Westerners will suffer some form of Lethologica during their lifetimes.'" Whatever does that statement mean? How many words do you have to forget to be lethological?
The word is from the Greek, like so many great things - from letho (forgetfulness) and logos (word). The disorder (for such it is) was first identified in 1913 by Jung, like so many other great things. Although I guess that in its non-pathological form, forgetting words is part of the human condition - one reason we make up so many new ones.