1. Marianne tells Elinor that Willoughby is presenting her with a really expensive gift: a horse. Even today, receiving such an expensive gift from a mere acquaintance would be inappropriate, so I won't belabor this point. I will, however, circle back to talk about the horse's name. Marianne doesn't really understand why the gift is inappropriate, but Elinor manages to talk her out of it based on the economic hardship to their mother that would accompany such a present.
2. Marianne tells Elinor that she knows Willoughby nearly as well as she knows Elinor, Margaret or Mrs. Dashwood - which is very well indeed.
3. Elinor hears Marianne tell Willoughby that she cannot accept the gift - and she tells him why, meaning that she divulges their economic situation to him (not mentioning the general impropriety, since that's not a point she concedes). Again, since people nowadays do not often divulge the particulars of their economic situation to mere acquaintances, this bespeaks a closer association.
4. In the same conversation, Willoughby calls Marianne by her first name, rather than observing the proprieties and calling her "Miss Marianne". If you've ever studied a foreign language, such as French, he has moved to the familiar/intimate "tu" form, and abandoned the socially proper "vous". Use of first names in this manner was something that occurred among family members (even extended family, such as in-laws, which is why Edward Ferrars is "Edward" and not Mr. Ferrars) and exceptionally close acquaintances. It was not the case in general, however.
5. In the same conversation, Willoughby promises Marianne that the horse is still hers, even if she may not yet take possession. He intimates that she will be leaving Barton for a more permanent establishment of her own, at which point the horse will be hers. Elinor's conclusion that he is obviously referring to a potential marriage between them in the future is entirely sound, even though we haven't heard of a proposal being made.
6. Margaret tells Elinor that Willoughby clipped a lock of Marianne's hair, which he kissed (*swoon*) before folding it inside a piece of paper, which he took with him. Although this particular custom has fallen out of favor, I am nearly certain that as soon as you give serious thought to this, you will recognize the extreme intimacy of such a gift. It was a custom at that time to have a lock of hair from a close loved one made into a piece of jewelry - often times a ring (for men) and a locket or a brooch (for women). Lovers often had one another's hair in a piece of jewelry, although sometimes hair was taken as a memento from a corpse and made into a piece of memorial jewelry. (You can consider it creepy all you like, but it is a personal and sentimental sort of token. And you can have a look at a piece of memorial jewelry that may well be made of Austen's own hair in this article in the LA Times.)
What about the horse's name?
Quoth Willoughby: "When you leave Barton to form your own establishment in a more lasting home, Queen Mab shall receive you."
Allow me to refer you to Act I, scene 4 of Romeo and Juliet, wherein Romeo and Mercutio have a conversation about Queen Mab, the fairy queen who is believed to drive around at night inciting love-related dreams. Mercutio describes her and her wee chariot in detail, as well as describing the variety of dreams she induces - she causes lawyers to dream of receiving fees, soldiers to dream of slitting throats, and lovers to dream of receiving love. In each case, Mercutio describes her as causing the very sort of dream that the dreamer most wants to believe in. (And Marianne most wants to believe in love, yes? And that Willoughby is going to marry her.)
But let's look at some more of what Mercutio says:
MERCUTIOWhen Willoughby says "Queen Mab shall receive you", Austen's readers would have known the stories of Mab to which Mercutio refers (as well as knowing of Mercutio's and Romeo's conversation). They would probably have all thought the Regency equivalent of "Hey now!" when they hit that line, since it evokes thoughts of the Rite of May (which involved sexual activities or connotations) as well as evoking dream-related notions. Let us hope that Marianne's hopes are not "vain fantasy" and that Willoughby proves more constant than the wind!
. . .This is that very Mab
That plats the manes of horses in the night,
And bakes the elflocks in foul sluttish hairs,
Which once untangled, much misfortune bodes:
This is the hag, when maids lie on their backs,
That presses them and learns them first to bear,
Making them women of good carriage.
This is she—
Peace, peace, Mercutio, peace!
Thou talk'st of nothing.
True, I talk of dreams,
Which are the children of an idle brain,
Begot of nothing but vain fantasy,
Which is as thin of substance as the air
And more inconstant than the wind, who woos
Even now the frozen bosom of the north,
And, being anger'd, puffs away from thence,
Turning his face to the dew-dropping south.
There they are, at dinner at Barton Park, and Margaret is goaded into revealing that there is a young man whom Elinor admires. If only Marianne hadn't taken up the conversation, it might have blown right over; instead, Margaret is goaded into revealing that his name starts with an F. Thank goodness for the insipid Lady Middleton, who doesn't care for high spirits, and for Colonel Brandon, "who was on every occasion mindful of the feelings of others". That sentence right there is one of the many ways in which Colonel Brandon proves himself wonderful, by the way. To give credit to other parties, however, Willoughby encourages Marianne to play the piano, thereby also contriving to turn the subject away from Elinor. It's nice to see people looking out for Elinor, who is (quite understandably) mortified.
Plans for tomorrow?
How about an outing? We'll all drive 12 miles down the road to see a wonderful home and grounds owned by Colonel Brandon's brother-in-law. Even though it's late October (in the story) and it's been very rainy, plans are made. Poor head-coldy Mrs. Dashwood will stay at home, but Elinor and Marianne will join Brandon and Willoughby and Sir John and Mrs. Jennings et al. for the party. It will be fun! Right? Well - we shall see tomorrow!