Green’s Dictionary of Slang

dizzy n.1

[Dizzy, the popular nickname of British prime minister Benjamin Disraeli, Lord Beaconsfield (1804–81)]

a clever man; esp. in phr. quite a dizzy.

[[UK]Kentish Chron. 22 June 3/4: Oh, Dizzy is a clever chap. / There never was a cleverer].
[UK]J. Ware Passing Eng. of the Victorian Era.