The use of "Sheila" to mean a 'young woman' (or simply, a female) in
Australian English (slang, like American "chick") may have derived from the
name of the Celtic Goddess "Sheela na Gig"...represented in carvings as an
"immodest woman" (see link for images).  And the term was imported to
Australia by Irish prisoners in the early days.  But...why wasn't it
adopted by those who came to the US?  Hmmm.

http://www.bandia.net/sheela/

There used to be a webpage apparently (Australian National Univ)...that
addressed this term in depth...but the link no longer works.  Darn.

Some dictionaries say that 'sheila' "has been driven out (of use) by
American English 'chick'.   Hmmm...

>From: George Myers
>
>Would I, however as thought further look "down under" as there a
>assignation of "Sheila" presents a similar enigma. I read and had
>confirmed by a recent immigrant form Oz (or Australia) that in fact a
>Sheila can mean a daughter-in-law and mother combination. I imagine
>referring to a woman, to a "mate," as a "Sheila" is some reference to
>both.