Green’s Dictionary of Slang

1. the lavatory.

1850		Fast Man 2:1 n.p.: ‘There’s another thing I have noticed—he’s never been down to the what’s-a-name yet.’ ‘And had currant pudding, too, for dinner,’ ejaculated Agnes.
1850		Fast Man 15:1 n.p.: ‘Go into—into—the—the what’s—a name,’ was my answer. The ‘what’s—a—name,’ was the water closet, and into it, poked Mr. Bowser.

2. (also whatsyname, wotsitsname) an unspecified object.

1890		‘Rolf Boldrewood’ Colonial Reformer I 52: His brow was wet with honest whatsyname.
1907		H. Lawson ‘The Song of the Doodle Doos’ in Roderick (1967–9 II) 236: O this is the song of I Told You So, and a ballad of Who’d Have Thought? / Of the Duty on Wotsitsname, Don’t You Know, and the Battle That Women Fought.
1947		R.S. Spears ‘A 22-Gun Ranger Walks’ Texas Rangers Jan. 🌐 [...] having stepped off the Panhandle stage where it stopped halfway between nowhere and what’s-its-name.

3. the penis.

1896		Farmer Vocabula Amatoria (1966) 113: Et caetera, m. The penis; ‘Mr. What’s-Its-Name.’.
1902		Lovely Nights of Young Girls (1970) 117: Let down my dress — oh, fie! for shame! / I won’t take hold of wat’s [sic] its name.

4. the vagina.

1896

Farmer Vocabula Amatoria (1966) 71: Comment-a-nom, m. The female pudendum; ‘the what’s-its-name.’.