Learn English – What does the slang “My eye!” mean

Theory 1: It's a contraction of "what are you up to" or "what are you doing". Basically, the last part (up to/doing) is completely dropped, and the rest is smushed together.

Theory 2: it's short for "what cheer", purportedly 17th century slang for "what's up".

http://www.urbandictionary.com/define.php?defid=1058973&term=wotcher

EDIT:

The MSN Encarta dictionary and freedictionary.com support Martha's second theory that wotcher is contraction of "what cheer". It is a slang (U.K.) that means the same as hello. It should be noted that it is not clear whether the slang is still in use anywhere in the U.K.

The meaning is most likely from this definition of fast:

a : wild
b : sexually promiscuous

From Wiktionary:

Having an extravagant lifestyle or immoral habits [from 18th c.]

And from Etymonline:

The sense of "living an unrestrained life" (usually of women) is from 1746 (fast living is from 1745).

Without delving too far into literary criticism, the preacher's daughter, Sabbath Lily, is often the victim of men in the novel and is later revealed to be a nymphomaniac. So this sense of the word is undoubtedly what was implied.