Explain Celia’s question to Rosalind—"Not a word?", in Shakespeare's As You Like It, at the start of Act I, scene iii.

In Act I, scene III, we meet Celia and Rosalind.
Rosalind's father has been banished by Duke Frederick (Celia's father), but Rosalind
herself has been saved because she and Celia love each other more than true
sisters:

The
other is daughter to the banish'd duke,
And here detain'd by her usurping
uncle,
To keep his daughter company; whose loves
Are dearer than the
natural bond of sisters.
(I.ii.290-293)

The women
learn of a wrestling match that will take place at Celia's house, on the lawn, and the
women attend. There they meet Orlando and try to keep him from wrestling, for they fear
for his safety and/or life. He says that there is no one in the world that would miss
him, but he appreciates their concern. He fights Charles and beats him. Afterward,
feeling connected to Orlando in that his sense of isolation is
similar to her own, Rosalind gives him her necklace to wear, for she has little else to
offer him in her reduced circumstances.

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ROSALIND.
Gentleman,

[Giving
him a chain from her neck.]

Wear this for me;
one out of suits with fortune,
That could give more, but that her hand lacks
means.--

The women leave.
This ends Act One, scene two. At the beginning of scene three, Celia says to
Rosalind:

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CELIA.
Why,
cousin; why, Rosalind;--Cupid have mercy!--Not a
word?

In this line, Celia is
asking Rosalind if she has nothing to say, or why she is so
quiet.

In the ensuing conversation, we learn that Rosalind
(daughter of Duke Senior) has a great deal on her mind, such as her father's predicament
and her own because of her father's banishment, and now she is also preoccupied by her
increasing thoughts Orlando, something which Celia asks Rosalind
about.

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CELIA.

...let
us talk in

good earnest: is it possible, on such a sudden,
you

should fall

into so strong
a liking with old Sir Rowland's
youngest

son?

It
is here, for the first time, that we learn that Orlando has been much on Rosalind's
mind, and that her feelings for him are growing.