In Chapter 1, while Enfield is gossiping with Utterson about Edward Hyde, he describes his sudden procurement of a check signed by a prominent figure in society:
For my man [Hyde] was a fellow that nobody could do with, a really damnable man; and the person that drew the cheque is the very pink of the proprieties, celebrated too, and (what makes it worse) one of your fellows who do what they call good.
The idiom, the “pink of the proprieties,” is a Victorian expression. Here, it describes someone who is absolutely beyond reproach, with a spotless reputation.
This idiom comes from an antiquated usage of the word “pink,” as something that is the perfect model of a quality or idea. To say something is “in the pink” of some quality is to say it embodies it to the highest degree possible. The similar expression, “in the pink,” usually refers to someone in the best physical health imaginable. Given the very physical nature of Jekyll’s affliction, this overlap carries some irony.
This idiom fits in with Stevenson’s naturalistic style of dialogue, which often relies on idioms, colloquialisms, and slang. Enfield, in particular, has a playful way of speaking; his use of slang and his sense of humor (he describes someone as “about as emotional as a bagpipe”) both add a dash of color to the story. The use of idioms, local humor, and modern (for the period) vocabulary situate this story firmly in Victorian London.