Rust (Wikimedia)
Rust, a plague of mankind since the advent of the Iron Age - with words for it that go back that far. Photo by Wikimedia
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The Hebrew word for rust, the reddish iron oxide coating that forms on wet iron exposed to air, is kha-lu-DA.

The word doesn’t appear in the Bible, but suddenly appears for the first time in the Mishna. That was redacted by Rabbi Yehuda HaNasi in around 200–220 CE, based on texts transmitted over generations by oral tradition.

In the tractate Kelim, we are given a list of many tools and instruments with different defects, each is designated ritually pure, unpure, or in dispute. One of these is “...a rusty needle, which slows down sewing, is pure”. The Mishna goes on and tells us that if it doesn't slow the sewing, it is impure. No explanation as to why is given. (Kelim 13, 5).

The sages of the Mishna didn't make up the word – it entered Hebrew from the Aramaic khalduta. The source of the Aramaic word isn’t certain but could well originate from the Greek word kelidos, meaning stain.

In the metaphoric sense, the word khaluda – as in rusty mind - has been in use at least since the High Middle Ages. The first known reference is in a letter written by Rabbi Jacob Anatoli to the Rambam in the first half of the 13th century: “Also with regards to brushing up my acquired tongue, from the days that had passed it has become rusty [khaluda]…”

Although the word khaluda was in regular use throughout the ages, in the 18th and 19th centuries at least two replacements were suggested by important writers, but never caught on. Baruch Linda used a noun form of the verb shakhak - to erode - in his highly influential book Reshit Limudim (“The Beginning of Learning”). “From this its look will change and shakhak (rust) will appear on it,” he wrote.
Several decades later, Samson Bloch, in his book Sheviley Olam (“Paths of the World”), the first modern geography book in Hebrew, used another word, khelah, which colloquially means "scum." “The land’s air will become wet and will raise khelah on metal tools,” he wrote.
In the early 20th century, as modern Hebrew was being born, the root kh-l-d was used to create a number of verbs and adjectives. For example, the author Mendele Mokher Sefarim wrote: “Deserted cars...sitting and rusting (makhlidot) in the corner”. For his part, the poet Haim Nachman Bialik created a high-brow adjective (non-poets would today use khalud) for rusty: “From over the walls their rusty (ha-khaled) weapons they lowered.”
Hebrew has a completely unrelated homographic root (that is spelled the same) meaning dig, which gave Hebrew a word for rat (khuldah) and naked mole-rat (kholed). But that we shall leave for another day.