でか

Coined as a cant word in the Meiji period. Derived originally from the term 角袖 (kakusode, “Japanese-style overclothes”, literally “square sleeve, corner sleeve”), in reference to the everyday clothing of plainclothes officers, as opposed to regular constables or police officers who would wear uniforms. The term kakusode was reversed by its constituent parts, producing sodekaku, and then abbreviated to produce deka.[1][2][3]

Although not explicitly recorded in dictionaries, the abbreviation process apparently involved a reformulation from sodekaku to kusodeka to include the term 糞 (kuso, “crap, shit”). This was then reanalyzed as kuso “crappy, shitty” + deka “detective”, with the latter part then being used independently.

The English term dick as slang for detective appears to be more recent, and thus had no influence on the formation of the Japanese term.

First attested in an argot dictionary of 1892.[1]

• (Tokyo) で​か [déꜜkà] (Atamadaka – [1])[2][4]
• IPA^(key): [de̞ka̠]

でか • (deka) 

1. [from 1892] (slang) a detective, a plainclothes police officer or constable

• 刑(けい)事(じ) (keiji)

• Entry at Gogen-Allguide (in Japanese)