Chastity Dagger

"Unappalled by the calm dignity of blameless womanhood, your minion has torn me from my spotless home, and dragged me, blindfold and shrieking, through hedges, over stiles, and across a very difficult country, and left me, helpless and trembling, at your mercy! Yet not helpless, coward sir, for approach one step -- nay, but the twentieth part of one poor inch -- and this poniard shall teach ye what it is to lay unholy hands on old Stephen Trusty's daughter!"

—Dame Hannah, Ruddigore

The villain is trying to have his way with the beautiful princess. He lifts up her dress and things look bad... but what's this? The maiden has quietly pulled a knife from her garter! She stabs him, putting an end to his evil ways once and for all.

The Chastity Dagger is the favored weapon of delicate, civilized young ladies in fantasy and historical works. It is almost always used as a means of self-defense when in danger of being raped, robbed or kidnapped, but a Femme Fatale may also use it in conjunction with her feminine charm to assassinate a male character while he is distracted.

Examples of the Chastity Dagger include:

Film

A double subversion in Robin Hood: Prince of Thieves; The Sheriff of Nottingham gives Maid Marian a dagger for protection. However she can look after herself and donates it to Robin's cause. Then, in the final battle when the Sheriff's in full on I Have You Now, My Pretty mode, Robin turns up and (after being disarmed) ultimately uses the dagger as a weapon of last resort to save her.
Romancing the Stone's Fake-Out Opening ends this way.
In Sherlock Holmes, thugs attempt to rob Irene Adler at knifepoint. Irene takes out a concealed knife and robs the robbers.
This was a favorite tactic of Milady DeWinter in The Three Musketeers 1993.
In Troy, Briseis stabs Agamemmnon this way during the sack of Troy.
Combined with Fan Disservice in The Emperor's New Groove: Yzma pulls up her skirt, causing the protagonists to recoil in terror. Turns out she keeps her dagger there.

Literature

Conan the Buccaneer has another version - all the virgins of Zingara carry small knives to kill themselves.
Discussed & subverted a couple of times in the Gor series; in Beasts of Gor Tarl strips a free woman and makes her comb her hair, where she has a poisoned needle hidden. At least one other time Tarl talks to a free woman about it too, saying she'd better not have a hidden dagger or it won't go well for her with her captors.
In Ruddigore, when Robin as Sir Ruthven Murgatroyd has Hannah kidnapped, she pulls a small dagger with which she menaces him in swashbuckling style. She releases it only to pick up a much larger sword and continues to advance on him as he cries for mercy.
Subverted three times in the Sienkiewicz Trilogy. The first woman chooses to stab herself instead; the second one gets easily disarmed, only for a henchman she had saved earlier to brain the villain with a blunt object; and the third has no dagger but uses a pistol as an improvised blunt weapon and deals a hit that costs the villain an eye.
In the Sword of Truth book Stone of Tears, Kahlan kills a would-be rapist with a ceremonial knife carved out of a human bone. She was half dead from poison at the time, and the book implies it wasn't her killing him, but the spirit of the man the bone used to belong to.
In Thais of Athens, a rich merchant tries kidnapping Thais' beautiful slave girl Eris. What he doesn't know is that Eris is an ex-priestess of a dark goddess and keeps enough hidden blades on her (near naked) body to leave him and his cronies Gutted Like a Fish on the ground before they even touch her.
In the Vorkosigan Saga, Barrayaran ladies traditionally carry "Vorfemme blades" (ornate concealable knives) for exactly this purpose.
- In Komarr this is lampshaded by discussing how Vor men get to have swords. In fact probably one of the best things one could have to fend off attempted rape is a good knife, as the attacker needs to close and the defender is likely to be if not necessarily off her guard, not courteously given time to prepare either.
In the Belgariad and sequels, Nadrak women have about four of them handy anytime.

Video Games

Touhou: Sakuya keeps a mind-boggling number of daggers in two bandoliers around her thighs.

Real Life

The Spanish Navaja in the hands of a woman is nicknamed "Salvavirgo" or "Virginity keeper". It was traditionally sheathed under the skirt so that if it was lifted without the owners consent the knife will be brought within easy reach. Spanish women in self-defense training were once taught before learning fancier tactics a few basic moves to keep a rapist away based on the assumption that a rapist obviously has to close with the victim.
- Salvavirgos often have cute slogans like, "I am devoted to my mistress", on them.
Kubatans are a modern version. They look like innocent keyfobs but they are effectively a hiltless stiletto hung on a keychain (usually rounded, or tapered at most rather then sharpened to get it past hoplophobic cops but a dagger like blow at the right place is sufficient to cripple someone). Reversing the grip one can also use them as the handle of a flail with the keys as the striker.
Good old fashioned mace.
Tasers.
Basically any of the street self-defense weapons popular in a respectable person's everyday carry could be used as a form of chastity dagger by analogy. Fighting knifes of course have not gone out of style, and when carried for this purpose they would not be just an analogy. This troper gets the impression that fighting knives are advertised more to a male demography but he could be wrong, and in any case there is no reason why a woman would not still have one as her choice for self-defense.