חָדַד (b. h.) [to cut, point,] to be sharp, pointed.
Pi. - חִידֵּד to sharpen, whet, point. Y. Bets. V, 63ᵇ top לחַדֵּד ראשו וכ׳ they differ as to pointing the top of the spit (on the Holy Day)—Trnsf. to whet the mind, to try somebody’s acumen, to puzzle. Taan. 7ᵃ (ref. to Prov. XXVII, 17) אף … מְחַדְּדִין וכ׳ so do two scholars whet each other’s mind &c. Naz. 59ᵇ, a. e. לא אמר … לחַדֵּד בה וכ׳ R. J. said it only in order to encourage the students in raising points; a. e.—Part. pass. מְחוּדָּד sharpened, well discussed, clear and ready. Kidd. 30ᵃ (ref. to ושננתם, Deut. VI, 7) שיהו ד"ת מְחוּדָּדִים בפיך that the words of the Law be ever ready in thy mouth (Sifré Deut. 34 מְסוּדָּרִים), v. גִּמְגֵּם.
Hithpa. - הִתְחַדֵּד to be whetted. Gen. R. s. 69 אין סכין מִתְחַדֶּדֶת וכ׳ a knife is whetted on the broad side of another, כך אין ת"ח מִתְחַדֵּד וכ׳ so is a student’s mind whetted by a fellow-student, v. supra.
Pa. - חַדֵּד 1) as preced. Pi. Sabb. 32ᵃ (prov.) נפל תורא חַדְּדֵיה לסכינא Ms. M. (ed. חדד, Ms. O. חַדְּדוּ) when the ox is thrown down, sharpen the knife (in critical moments man’s sins are visited, v. חִגְּרָא). Ḥull. 43ᵇ, a. fr. לחַדּוּדֵי וכ׳ to try Abbayis’ acumen.—Part. pass. מְחַדַּד ready in answering questions, well-versed, quick (v. preced.). Erub. 13ᵇ האי דמְחַדַּדְנָא מחבירי Ms. M. (ed. מחבריא) the reason that I am readier than my fellow-students. Yeb. 14ᵃ ב"ש מְחַדְּדֵי טפי those of the school of Sh. were more acute. Nidd. 14ᵇ מחדדי שמעתתיה (read: מְחַדְּדָן), v. infra. —2) to cheer up, entertain. Gitt. 68ᵇ, v. חַדְוְותָא.
Ithpa. - אִיתְחַדַּד to be well studied, ready at hand. Keth. 62ᵇ [read:] מִתְחַדְּדָן שמעתתיה (Rashi: מחדדן) he recited his lessons (traditions) well.
So a man sharpens the wit of his friend.
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JPS adds a comment that what is translated as "wit," panim [in construct form], literally means "face." Some translations pick up on this by saying the friend's "countenance" is sharpened.
"Iron sharpens iron, and a man sharpens the person of his neighbor."
-- Contemporary Aramaic in Plain English trans.
Iron together with iron,
and a man together with his friend. -- Robert Alter (Norton, 2010)
Alter's note:
Iron together with iron. This is usually understood to refer to magnetized iron, which clings to iron, and so does a man to his friend. The force of the proverb is in its terrific compactness, which the translation tries to preserve.
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Alter rarely cites his sources, and his claim that his reading is "usual" is belied by the lack of commentary or translation on Sefaria that mentions any reading OTHER than "sharpening." All four English translations on Sefaria use "sharpen" for the verb. The Targum similarly uses "sharpen" -- see below. Of the 38 translations into English on Bible Hub, in addition, only two -- both from the 16th Century -- say anything other than that one "sharpens" either the other individual or the other's face/countenance. Those translations include the old and new JPS. So that makes a to total of 38 of 40 that speak of sharpening, and not one that talks of iron magnetized and sticking to iron. Might be lots of such translation and commentary, but Alter doesn't cite any, so it's hard to understand his reading, however beautiful, as "usual."
Here is one contemporary Christian author, who writes on Ancient Hebrew poetry, reviewing some alternative readings of the verb yud-chet-dalet [יחד] as related to hod, as in "to make joyful," or as related to yachad, "unite, singular," instead of "חדד" to sharpen. Unlike Alter, this teacher cites and discusses sources. (https://ancienthebrewpoetry.typepad.com/ancient_hebrew_poetry/2009/12/another-look-at-proverbs-2717.html)
Here are the two translations using "comfort" instead of "sharpen," just for the record:
"Like as one yro whetteth another, so doth one man comforte another." -- Coverdale Bible, 1535
"Like as one iron whetteth another, so doth one man comfort another." -- Bishops Bible, 1568
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ותחזק מצרים על העם. אחר שלקו בעשר מכות הוצרכו לשלחם בעל כרחם ועוד שנטלו מהם כל ממונם זהו שאמר מיד וינצלו את מצרים, וכן דרשו רז"ל במדרש משלי מאי דכתיב (משלי כ״ז:י״ז) ברזל בברזל יחד ואיש יחד פני רעהו, ברזל בברזל יחד זה משה הצדיק ופרעה הרשע שהיו מתנקשים זה עם זה בדברים ...
כך עשה הקב"ה למצרים לקו ושלחו את ישראל ונטלו את ממונם מי גרם להם על שנתקשו לפני הקב"ה אף הוא נתקשה להם על ידי משה שנאמר ברזל בברזל יחד.
... When Solomon speaks (Proverbs 27,17) about “as iron sharpens iron, so a man sharpens the wit of his friend,” the words “iron sharpens iron” refer to Moses the righteous and Pharaoh the wicked respectively. They were opposing each other with words....
[very long story of opposition]
Pharaoh’s obstinacy was countered by Moses’ obstinacy. This is what Solomon had in mind when he spoke of iron sharpening iron.
So does one man’s heart to another.
This verse can also relate to Eruvin 13b above.