This expression might be the oldest fruit in the French book. "To fall in the apples" is the literal English translation, but it's more than that. If you fell into a bunch of apples by slipping on a fallen crate, what might your outcome be? That's right: a serious "black out."
Tomber dans les pommes means to faint, pass out, loose consciousness. You might also hear the variation partir dans les pommes, and the equally informal expression rester dans les pommes - "to remain unconscious."
There is no definitive explanation of how this idiom was formed, just tentative ones. The first is that it may be a distorted version of tomber dans les pâmes, related to se pâmer, an old-fashioned word for swooning/fainting. Another theory is that it is derived from an expression used by writer George Sand in a 1830 letter to her mother. Explaining that she was exhausted, she described her state as “being in baked apples." "Je suis un peu dans les pommes cuites" she wrote. This latter explanation is somewhat far-fetched, considering it involves a shift in both form and meaning, but then again, the French like their fruity idioms.
Alternatively, you could use the Québec synonym faire l'étoile de mer - "to do the starfish," which is very pretty and makes so much more sense!
N'ayant rien mangé depuis 12 heures, elle est tombée dans les pommes. Not having eaten anything for 12 hours, she passed out.
Quand j’ai vu le sang, je suis tombé dans les pommes.
When I saw the blood, I fainted.