• says her hair grows "so fast." Her hair probably grows at about the same rate as any other young woman's hair. But once she gets over the initial shock of seeing herself in that pier-glass without her once bountiful head of hair, she will get accustomed to her appearance and accept the consequences of her decision. Gradually her hair will grow back. In fact, she might even finds that she likes wearing it shorter. For one thing, she won't have to spend several hours washing and drying it.

Della and Jim Young are a young couple and the primary characters in "The Gift of the Magi" by O. Henry. They were never rich, but now Jim is making even less money, so there is none to spare. It is Christmas Eve, and Della is distressed because she wants to get the husband she loves so much a nice gift but has no money. Well, she has one dollar and eighty-seven cents, but that is certainly not enough to buy Jim a watch fob worthy of his prized possession: a pocket watch passed down to him by his father and his grandfather.

In order to accomplish her goal, Della sells her own prized possession: her long, luxuriant hair. The money she gets from her sacrifice goes to buy Jim's gift; however, she is afraid of his reaction when he comes home tonight and sees that she has cut off her hair. 

Jim is, indeed, surprised when he sees her, but of course that is because the gift he got her (by selling his pocket watch) is now useless.

For there lay The Combs--the set of combs, side and back, that Della had worshipped long in a Broadway window. Beautiful combs, pure tortoise shell, with jewelled rims--just the shade to wear in the beautiful vanished hair. They were expensive combs, she knew, and her heart had simply craved and yearned over them without the least hope of possession. And now, they were hers, but the tresses that should have adorned the coveted adornments were gone.

But she hugged them to her bosom, and at length she was able to look up with dim eyes and a smile and say: "My hair grows so fast, Jim!"

Della is speaking literally here, telling Jim that her hair will grow back quickly and she will soon be able to use the lovely combs he got for her. Until then, the combs are as useless to her as the watch fob is to him (since he no longer has a watch). 

When Della first declares that her hair grows “awfully fast,” her goal seems to be to reassure Jim that her current appearance will not last forever. Her hair is quite short now that she has sold the bulk of it, and she has curled it so that it lies close to her head. She must look quite different from how she did before, and so she is very concerned that Jim will not think she is beautiful anymore, that he might be somewhat less attracted to her without her beautiful, long, chestnut hair.

When Jim first sees her, his shocked reaction to her appearance makes Della grow even more concerned. After she receives Jim’s gift, a beautiful set of hair combs that she will not be able to use with her short hair, she understands Jim’s baffled and incredulous response. She then assures him a second time that her “hair grows so fast” in order to convey the idea that she will certainly be able to use the combs someday soon, because her hair will grow back quickly. She does not, one might imagine, want Jim to feel that his gift is useless or superfluous, and so she tells him how quickly her hair grows in order to cheer him with the thought that she will be able to wear the combs sooner rather than later.