The full name of the current Hebrew month is Mar-Cheshvan, and the abbreviated version is Cheshvan.
My professor of Semitic Studies was of the view that Mar-Cheshvan is linguistically a mixed-up version of an original Babylonian name, yerach sh’mini, which means “the eighth month” in Hebrew. If you count the months starting with Nisan it works out that Tishri is the seventh month, which is how the Torah describes it, and Mar-Cheshvan is the eighth.
One tradition however says that because the month has no festivals or even
fasts it was mar “bitter.” This is said to explain why the name has become literally Bitter Cheshvan.
The original Biblical Hebrew name for the eighth month was Bul, “rain showers,” (I Kings 6:38) as Cheshvan is when the winter rains begin. One view says that the phrase mar mid’li, “a drop in the bucket” (Isaiah 40:15) may have contributed to the modern name of the month.