Oh, yeah! That might be the language barrier, because it’s a pretty simple joke, really, but it’s also about sex, so that always weathers translation roughly (mostly because everybody’s giggling too much to explain).
Mild spoilers under the cut.
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What we see: Sherlock pulls a pair of handcuffs out of his pocket and claps them on the bad guy. This implies that Sherlock carries a pair of handcuffs around as a matter of routine. Janine, quite reasonably, expresses her surprise.
Because carrying handcuffs is weird if you’re not a cop. WE know Sherlock is a Consulting Detective, and that they’re a reasonable thing for him to carry in his line of work. But for many of us common gutter-minded individuals of the world, we also know that handcuffs are sometimes used as a sex toy.
We also know that there is only one of Sherlock vs. many many kinky bastards in the world.
So when Sherlock says “Down, girl,” he is poking fun at the idea that Janine’s mind might have gone to sexy bondage fun-time first.
There’s plenty of possible subtext there for fandom to analyze, like questions such as, “But maybe Sherlock IS a kinky BDSM bastard?” and “But he wasn’t actually EXPECTING a crime at the wedding, so why did he have a pair of handcuffs on that particular day?” and “Is Janine just surprised, or is she into the idea?” I leave that work to others. ^_^
Hope that it helps!
(Disclaimer: don’t use police-style handcuffs for sex. You’re hosed if you lose the key, and the metal rims can easily cut and abrade a person’s wrists during sexual activity. Also in a lot of SSC bondage, the person who is tied up should be able, in an emergency, to get out on their own.)
