We learned there: one who says "konam," "konach," or "konas" has used substitute terms for "offering." Regarding these substitute terms, Rabbi Yochanan said they are a foreign tongue. Rabbi Shimon ben Lakish said they are language the sages devised so that one might vow by them, and likewise it says, "in the month which he had devised from his own heart" (1 Kings 12:33). And what is the reason the sages instituted substitute terms? So that one would not say "offering." But let him say "offering"? Perhaps he will say "an offering to the LORD." But let him say "an offering to the LORD"? Perhaps he will say "to the LORD" and not say "offering," and so bring forth the name of Heaven in vain. And it was taught, Rabbi Shimon says, and so on.
Why the Sages Coined Substitute Words for Vows
Yalkut Shimoni on Torah 435:2
תְּנָן הָתָם הָאוֹמֵר קוֹנָם קוֹנָח קוֹנָס הֲרֵי אֵלּוּ כִּנּוּיִים לְקָרְבָּן, כִּנּוּיִין רַבִּי יוֹחָנָן אָמַר לְשׁוֹן גּוֹיִים הֵן רַבִּי שִׁמְעוֹן בֶּן לָקִישׁ אָמַר לְשׁוֹן שֶׁבָּדוּ חֲכָמִים לִהְיוֹת נוֹדֵר בּוֹ, וְכֵן הוּא אוֹמֵר (מלכים א יב, לג) "בַּחֹדֶשׁ אֲשֶׁר בָּדָא מִלִּבּוֹ". וְטַעְמָא מַאי תָּקִינוּ רַבָּנָן כִּנוּיִין, דְּלֹא לֵימָא קָרְבָּן. וְלֵימָא קָרְבָּן, דִּלְמָא יֹאמַר קָרְבָּן לַה'. וְלֵימָא קָרְבָּן לַה', דִּלְמָא יֹאמַר לַה' וְלֹא יֹאמַר קָרְבָּן, וְקָא מַפִּיק שֵׁם שָׁמַיִם לְבַטָּלָה. וְתַנְיָא רַבִּי שִׁמְעוֹן אוֹמֵר וְכוּ'.