Green’s Dictionary of Slang

tiggy n.

[? ’tec n. (1) or game of tig, a juv. catching game]

a detective.

[UK]E. Pugh Spoilers 5: It shows you ain’t too anxious – not just at present – to be recognised by the tiggies.
[UK]E. Pugh Cockney At Home 281: You seem to know a lot for a four-eyed man. Ain’t a tiggy, are you?