What does a 2nd century Jewish jurisprudence text and a 13th Persian mystic poetry have in common? At first glance, not much, but there is more linguistic commonality than first meets the eye.
The Jewish Mishnah
The Mishnah is a recording of Jewish oral law collected between 70 CE and 200 CE by many generations of Rabbis. Its name means "repetition".
The etymology of the term comes from the Semitic root TH-N. In Hebrew, the TH sound in other Semitic languages becomes a SH sound. For example, Hebrew for the numbers 2 and 3 is Shnyim and Shalosh, while the Arabic equivalent is ITHNAYN إثنين and THALATHA ثلاثة.
So, if we were to write Mishnah in the more common Semitic form in Arabic letters, it would be approximated to: MITHNAH مثناه or مثنى. This last form appears in the Quran three times, pronounced MATHNA and spelled the same as مثنى when having no diacritics (Tashkeel), and meaning "two" or "dual".
Jalaludin Rumi's Masnavi
Masnavi is the magnum opus of Jalaludin Rumi, a Persian Sufi master who lived in Asia Minor. Its full name is Masnavi e-Ma'nawi مثنوي معنوي. It is composed in couplets, where the second half of the verse rhymes with the first from which it derives its name.
The name "Masnavi" is the Persian pronunciation of the Arabic word Mathnawi. The TH sound is rendered as S (much like modern spoken Arabic dialects, e.g. in Egypt), and W is rendered as V, a common thing in many languages (e.g. Polish, Farsi, Hindi). The couplet rhyme derives from the Arabic THANI (ثاني second), also from the same Semitic root: TH-N.
Other interesting derivation of the same Semitic root is are Muthanna مثنى meaning second, and a common ancient Arab male name. One such person is Al-Muthanna ibn Haritha, who lends his name to the modern governorate of Al-Muthanna in Iraq. It is also the modern word for the grammatical number of 2 in Arabic, in addition to the singular and plural forms.
The content of the two works are vastly different, although both are religious in nature and origin. However, the non obvious etymology is much closer than one thinks at first.