brain freeze
noun phrase

- a sudden failure to remember a basic fact, name or word, especially when this is embarrassing or in public

- a sudden severe headache caused by eating or drinking something very cold too quickly (also ice-cream headache, cold-stimulus headache, sphenopalatine ganglioneuralgia)

Oxford Advanced Learner's Dictionary

—
Most of us have experienced the sudden forgetting of a word or phrase, sometimes a person’s name, or a place, or a book, a film, an object, or even a concept.

The psychological term for this experience is “tip of the tongue” state (TOT-state). Experimental psychologists in the 1960s showed that people in a TOT-state were able to access information about letters, sounds, and meanings related to the word they were searching for, even when they couldn’t come up with the word.

The fact that we suddenly remember (or at least recognise) a forgotten word, suggests that the memory is still present but in the heat of the moment we are unable to retrieve it.

Forgetting a word can be frustrating, but most of the time the situation resolves itself quickly. The word comes back, and we continue. A study by Burke et al (1991) found that most tip of the tongue states cured themselves spontaneously without too much trouble.

—
SYNONYMS for brain freeze

clean forget, completely fail to remember, consign to oblivion, draw a blank, escape one’s memory, let slip from memory, lose consciousness of, lose sight of

—
SMUGGLE OWAD into a conversation, say something like:

“Just in that moment Jim experienced a total BRAIN FREEZE: he knew who she was—he just couldn’t remember her name.”