The Fashion in shrouds

I came to find the circumstances in which appeared the phrase "It's crackers to slip a rozzer the dropsy in snide," only to discover it was no circumstances at all. The phrase was an example of a phrase. I wish I knew how someone in 1938 would have heard that example because much of what was said back then is heard as offensive in 2019, to judge from the other reviews I've seen here. Were they taken literally back then? As an actual detective story, perhaps the kinds of plot twist were original back then but we've seen a lot of them since then. Ms. Allingham is an interesting stylist (in the writing, not the fashion sense), enough so to make this historical document readable today.