From Wiktionary {but doesn't give a derivation, though I love your suggestion Topsie}
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Pronunciation
IPA: /tɒti/
Rhymes: Rhymes:English:-ɒti
[edit]Noun
totty (uncountable)
(UK, slang, English) sexually attractive women considered collectively; usually connoting a connection with the upper class.
(slang, English) an individual sexually attractive woman  [quotations ▼]
[edit]Usage notes
Although denoting a countable subject, the noun is a mass noun. A single person is described as "some totty" or "a bit of totty". But a group of people can also be referred to as "some totty" or "the totty".
[edit]Related terms
top totty
hotty
[edit]Synonyms
talent