Two possibilities occur to me:
(a) He’s so stupid he doesn’t get the sexual connotation of finger-sucking (i.e., he expects her to suck his fingers only), and
(b) he’s dickless.
Reminds me of the [ETHNIC] who was arrested for indecent exposure.
Someone asked him to count to eleven. 
Read the article that I linked to in my first post in this thread, then watch the scene in my next post, and then tell me that, in a film that refers to fellatio, premature ejaculation, pornographic photos, a stuffed beaver, and sex with appliances, where there’s an object that is undeniably a reference to a sexually charged scene in a classic film, you still think that the joke consists of a bracelet somehow going from a woman’s wrist to her ankle.
That’s the joke.
Cool. Not exactly what I had in mind (I meant a video.), but it does tell me something.
It does look like a bracelet to me. Not a typical anklet, which is usually has more chain to it. (Google image search)
I can’t imagine that thing as a garter. Ouch. It would need to be lacy-looking for that.
I don’t think the garter thing works at all, so that’s out. But I still can’t rule out the bracelet idea or her being intentionally flirtatious to make him think about her genitalia. Or possibly even the absurdity of it being genital jewelry.
I really do need to see the scene to give my full opinion, which apparently at least one poster wants.
Okay, I found the scene. I found the whole movie online, and then used a transcript to figure out where it was in the movie.
My verdict? She is flirting with him hardcore. The bracelet idea doesn’t make sense in that context. She is trying to make him think about where such a thing might slip down from.
It is all innuendo and not absurd humor–the anklet is just an anklet. She’s just flirting.
The only thing that seems like absurd humor in the exchange is a reference to tomorrow being Arbor Day and the lady needing her rest. But, in this context, I think it’s probably a joke about wood.
Sorry DH.
Okay, I can accept that alternative explanation. Sort of. 
FWIW, here’s Carol Burnett’s parody of Double Indemnity in which the anklet is spoofed: Double Calamity.
Don’t worry, I’m not that invested in it, but I still think the anklet/bracelet thing is sillier and funnier, its just a pity we don’t see what she does with it after she removes it 
