כל המסתכל באצבע קטנה של אשה כאילו מסתכל במקום התורף -- Anyone who gazes upon a woman’s little finger is considered as if he gazed upon her naked genitals (for if his intentions are impure, it makes no difference where he looks or how much is exposed; even less than a handbreadth. - Steinsaltz)
Other uses of a similar root תרף include Bereishit 31:19, where Rivka steals the Terafim from her father Lavan. There, the Terafim are understood to be some form of idolatrous or magical device.
Jastrow defines תרף as to make soft (seems more related to רפה), and תרפות as obscenities (which seems to be used across the board as a reference to idolatry). He defines תורף as "that which makes a debt collectable", and תורפא as a valid claim.
Tanchuma says the idols of Lavan are called תרפים to convey a sense of dirtiness and impurity.
The Mishna in Terumot 8:8 uses the term מקום תורפה to describe "a hefker place where [the barrel] will be destroyed" (acc. to Bartenura ad. loc.)
In B. Shavuot 16a the Gemara refers to the upper fields of the Mount of Olives as תורפה של ירושלים היתה ונוחה היא ליכבש, a vulnerable point of Jerusalem from which it would be easy to then take the rest of the city. Rashi there explains: "גלויה וערותה מקום", which just brings us full circle.
All these related usages of תרף seem untenable as clear definitions of female genitalia. I have yet to see anyone explicitly explain this usage. Does anyone have any sources explaining the term? And secondly (as a bonus), why does our Gemara in question not use a more classical description, such as ערוה or אותו מקום?