The word אַף is often used as a noun meaning "nose", "nostril", or "anger", but here it is used in a different way. The Scholars Gateway parses it as a conjunction, and my iPhone app parses it as a particle. Both the website and my app list "also" and "yea" as definitions, but neither seems to fit the bill. Here is a comparison of translations which offers a couple interpretations.
וְהַנָּחָשׁ֙ הָיָ֣ה עָר֔וּם מִכֹּל֙ חַיַּ֣ת הַשָּׂדֶ֔ה אֲשֶׁ֥ר עָשָׂ֖ה יְהוָ֣ה אֱלֹהִ֑ים וַיֹּ֙אמֶר֙ אֶל־הָ֣אִשָּׁ֔ה אַ֚ף כִּֽי־אָמַ֣ר אֱלֹהִ֔ים לֹ֣א תֹֽאכְל֔וּ מִכֹּ֖ל עֵ֥ץ הַגָּֽן׃
LXX - ὁ δὲ ὄφις ἦν φρονιμώτατος πάντων τῶν θηρίων τῶν ἐπὶ τῆς γῆς ὧν ἐποίησεν κύριος ὁ θεός καὶ εἶπεν ὁ ὄφις τῇ γυναικί τί ὅτι εἶπεν ὁ θεός οὐ μὴ φάγητε ἀπὸ παντὸς ξύλου τοῦ ἐν τῷ παραδείσῳ
Vulgate - sed et serpens erat callidior cunctis animantibus terrae quae fecerat Dominus Deus qui dixit ad mulierem cur praecepit vobis Deus ut non comederetis de omni ligno paradisi
YLT - And the serpent hath been subtile above every beast of the field which Jehovah God hath made, and he saith unto the woman, `Is it true that God hath said, Ye do not eat of every tree of the garden'
The LXX and Vulgate translate אַ֚ף as "why" (using the interrogatives τί and cur respectively). The YLT uses אַ֚ף to form a yes-no question ("is it true that..."). Which interpretation is correct, if any? How would you define אַ֚ף in this context? And what is that punctuation mark beside the patach?