I have heard that English has a word for the situation in which you are talking to an acquaintance when another acquaintance walks by whom you greet. Neither knows the other so you are about to mutually introduce them, but you suddenly realize that you do not remember either of their names!
What is the word or phrase for that? I have consulted books of idioms, a thesaurus, and multiple search engines without any luck.
Perhaps you're referring to the idiom mind went blank.
Edit: I'm assuming you're referring to the sudden inability to recall information that you already know, in general. This could be someone's name (as in the example you provided), or it could be, say, the sudden inability to remember some factoid during an exam.
If I'm wrong about that assumption, however, there's this word:
lethonomia: 1. A tendency to forget names. 2. The inability to recall the right name.
It seems that people involved in the study of memory, lexical retrieval, and cognitive psychology call these tip-of-the-tongue experiences. This is an interesting article from New Scientist. Here's a brief excerpt:
More likely, tip-of-the-tongue experiences occur when we're trying to recall rarely used words, Pyers says. (Note: The article says Pyers is a psychologist at Wellesley College in Massachusetts.)
"People often have tip of the tongue experiences for proper names, again because we don't use them very frequently," she adds. *
(*Inconsistent hyphenation of tip-of-the-tongue experiences in original.)
Not remembering names is called a tartle. It's a word from Scotland.
While previously-suggested jamais vu is somewhat related, I think presque vu (from French for "almost seen") is a more accurate term for momentarily forgetting known names or words. While jamais vu is "the phenomenon of experiencing a situation that one recognizes but that nonetheless seems very unfamiliar", presque vu is "failure to retrieve a word from memory, combined with partial recall and the feeling that retrieval is imminent." (Of course, a problem with presque vu in the example is that "you suddenly realize that you do not remember either of their names" is incongruent with "the feeling that retrieval is imminent".)
If you are talking about forgetting something which you surely knew all along, the word is jamais vu: the experience of being unfamiliar with a person or situation that is actually very familiar. As you would guess, this is complementary to the more famous deja vu.
Instead if you want to refer to the people whose names you have forgotten, you may use thingummy, which refers to something unspecified whose name is either forgotten or not known.
