—i.e. D for a detective. Look sharp! the dees are about.
Dee (Dr. John). A man of vast knowledge, whose library, museum, and mathematical instruments were valued at pound £2,000. On one occasion the populace broke into his house and destroyed the greater part of his valuable collection, under the notion that Dee held intercourse with the devil. He ultimately died a pauper, at the advanced age of eighty-one, and was buried at Mortlake. He professed to be able to raise the dead, and had a magic mirror, afterwards in Horace Walpole's collection at Strawberry Hill (1527-1608).
Dee's speculum or mirror, in which persons were told they could see their friends in distant lands and how they were occupied. It is a piece of solid pink-tinted glass about the size of an orange. It is now in the British Museum.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894