There is, dear reader, a precise word for not being able to remember the precise word: lethologica. This was sometime a paradox, but next time you misplace the mot juste, comfort yourself with the fact that you are simply having a lethological moment.
Lethologica was invented by Carl Jung and is simply a combination of lethe - forgetfulness - and logica- wordy. In Greek mythology there was a river of forgetfulness in the underworld called Lethe. When you bathed in Lethe you forgot everything and were washed in sweet oblivion. That's why Keats opens Ode to a Nightingale with:
My heart aches and a drowsy numbness pains
My senses, as though of hemlock I had drunk,
Or emptied some dull opiate to the drains
One minute past and Lethewards had sunk.
Similarly, when Hamlet's dead daddy reveals that he's been murdered, he's pleased to see that Hamlet gets antsy:
...duller shouldst thou be than the fat weed
That roots itself in ease on Lethe wharf,
Wouldst thou not stir in this.
There's also some poem where Time sits on the banks of Lethe throwing people's names in to the water, but I can't remember what it is. Perhaps it's Dante. It sounds like Dante. Anybody know? I just can't remember.
Jung tries to remember that splendid word he thought up.