Green’s Dictionary of Slang

1. a need to urinate.

1614

Jonson Bartholomew Fair III vi: No, I have a very great what-sha-callum, John.

2. (also what shall’s callum) anyone for whom one cannot provide the name; also attrib.

1598		Jonson Every Man In his Humour I ii: Oh, Musco, didst thou not see a fellow here in a what-sha-call-him doublet; he brought mine uncle a letter even now?
1664		C. Cotton Virgil Travestie (1765) Bk I 44: A hardish Kind of Name it was, / Where once your what shall’s call’ums (rot em, / It makes me mad I have forgot ’em) / Liv’d a great while.
1824		(con. 17C) W. Scott Redgauntlet (1827) 274: Come, patron, we will drink to Mr What-shall-call-um – What is his name?