That term you’re searching for might be on the tip of your tongue, but most of the time, all you can do about it is stew until you either: a) have an epiphany, or b) use some nonsense word like whatchamacallit and move on.
Source: Science of Us, June 12, 2017.
Researchers refer to this “tip-of-the-tongue phenomenon” indicating it’s not quite as simple as just blanking out. “Words are not atomic units as is sometimes assumed,” explains linguist Chi Luu. You still know everything about that thing you’re trying to describe, excerpt for how to form the sounds that allow you to communicate it to someone else.