On the tip of my tongue 

knowing something that cannot immediately be recalled, like a well-known word or familiar name. When experiencing it, people feel that the blocked word is on the verge of being recovered. Despite failure in finding the word, people have the feeling that the blocked word is figuratively "on the tip of the tongue."

I can't remember the word for eating pussy, but it's right on the tip of my tongue.
I can't remember the word for licking balls, but it's right on the tip of my tongue.
I can't remember the slang word for "anus," but it's at the tip of my tongue.

On the tip of my tongue by derbyjames January 1, 2010