We previously discussed a number of words with the root סכן. One word that might seem missing in that discussion is sakin סכין - "knife". However, while this is how it is spelled in Rabbinic Hebrew, and the Aramaic cognate is סכינא, in Biblical Hebrew the word is spelled with a sin, not a samech - שכין. It appears only once, in Mishlei 23:2.
Klein points out that שכין derives from the base שכך, which he defines as "to be pointed, to transfix" (and is not related to the homographic root שכך meaning "to cover, lay over"). Two other unique Biblical words come from this root: sech שך - "thorn" (appears only in the plural, sikim שכים in Bamidbar 33:55), and suka שכה - "barb" (in Iyov 40:31). Also related is the post-Biblical sika סיכה - "pin, peg, brooch".
Chaim Rabin, in his article מילים זרות ("Foreign Words") in the Encyclopedia Mikrait, writes that sakin belongs to a group of words that entered Hebrew from Asian languages that were neither Semitic or Indo-European. He doesn't say where sakin came from, but points out that in the Lexicon of Hesychius, we find the Greek word συκινη meaning "sword". However, without any further information, and in light of the convincing etymology that Klein (and others) provide, I'm not so inclined to accept this theory.
Although the older dictionaries point out that sakin is a feminine noun, both Avineri (Yad Halashon, 420) and Sivan (Better Hebrew Usage, 232) point out that in Talmudic Hebrew we find that it appears both as masculine and feminine (and Avineri writes that Rashi and the Rambam use it in the masculine). Since the word "sounds" masculine, there is no reason to insist on it being feminine. However, Google still supports the dictionaries: 14,800 hits for "sakin gedola" סכין גדולה vs only 2,040 results for "sakin gadol" סכין גדול.