There's a word in the Hebrew Bible, selah (סלה), for which commentators have proposed multiple interpretations. That's usually a sign that no one really knows what it means.
The word selah occurs three times in the book of Habakkuk and seventy-one times in the book of Psalms. Each time it shows up, it seems to mark some kind of break in the flow of what's being said. (For a comprehensive look at what's been proposed, check out the Wikipedia article on selah.) It seems to be somehow related to the musical nature of the Psalms as they were originally performed, such as a musical interlude. (Prophets like Habakkuk also performed their prophesies musically, as you can see from the description of Ezekiel as a singer in Ezekiel 33:32.)
The hero David was known to play a mean harp in his day (see 1 Samuel 16:23). When he has to flee from King Saul, one of the places he sojourns is the
(1 Samuel 21). land of Philistia
We know very little about the Philistines and their language, though there is increasing consensus that the Philistines could have been an Indo-European speaking people who settled on the coast of the
Mediterranean as part of the "Sea Peoples" phenomenon of the 2nd Millenium BCE. (For a full study of Philistine language, see the Wikipedia article on the topic.)
Whenever peoples live beside each other, they may fight but they also exchange products and, eventually, words. I am proposing that the word selah is a borrowing from the Philistines. Now, I'm not saying that King David himself is the one that brought it back or even that he necessarily wrote all of the Psalms attributed to him. (Though it does occur in many Psalms attributed to him [e.g., Psalms 3, 4, 9, 20, 21 et al.].)
So here's my theory. If Philistine is Indo-European and the word selah is a borrowing from them, it could be an etymological cognate to the word "solo." (The Proto-Indo-European root *solw- carries the meanings of "whole" and "alone" [cf., Latin solus 'alone' and Irish slán 'safe'].)
Musical notation is precisely where we tend to see word borrowing and then a tenacious preservation of terms. Notice how we still use Italian terms such as fortissimo and crescendo.
The possibility that selah indicated a musical interlude is not a new assertion. What I am saying, however, is that the word actually is related to the Italian word which we use even today to describe a musical solo.
Another example of how musical words tend to be borrowed, not created, is found in the word for "harp" or "lyre." What David played on is called, in Hebrew, a kinnor (כנור). But the Greeks called their instrument a kithara. (The word may be also related to the Old Persian sithar.) The Romans would borrow the word as cithara. It was borrowed by Semitic speaking peoples and shows up in Daniel 3:5 and the Odes of Solomon 6:1. It made its way into Arabic and then came back to
Europe as … "guitar!"