Selah (סֶלָה, or סֶּלָה if it falls after a naked vowel) is a word of much mystery. We say the word “selah” often enough during the day to want to understand its meaning.
For instance, in the Kedushat Hashem section of the Amidah (otherwise known as the bottom of the first page) we say to the Almighty:
.אַתָּה קָדוֹשׁ וְשִׁמְךָ קָדוֹשׁ, וּקְדוֹשִׁים בְּכָל־יוֹם יְהַלְלוּךָ סֶּֽֽלָה
No one really knows what the word “selah” means. The presumption nowadays is that it is either an interjection or a note for the choir director.
It appears 71 times in 39 of the Psalms. It also appears thrice in Habakkuk chapter iii, Habakkuk’s prayer section. In many of the Psalms there are notes at the beginning to the conductor. (Habakkuk may also have had a conductor.) Thus scholars have deduced that these insertions of “selah” may be indications for a pause.
Some scholars have played with the word salal (סלל) which means to “mound up” as one might mound the gravel for a road bed. (We note, though, that it is not to be confused with the word for “rock,” סֶלַע.) This adduction led those scholars to opine that the word instructed the choir to lift up their voices (to a higher volume, a higher pitch, or a higher mound, we presume). Some have even produced a further notion, saying maybe it is an acronym - “siman lishnot hakol,” “sign to change the voice,” or “sov lema’alah hashar,” “turn up the singer.”
Some do continue to believe that as an interjection or even an adverb it provides emphasis. They feel when we say “amen selah” we mean to emphasize the “amen.” In Eruvin 54a 14-15, one rabbi made a questionable case, quoting only from Psalms 48:9 “… ad olam selah,” inducing (and, we feel, unduly extrapolating) that since “selah” modifies “forever,” it must itself mean “forever.”
We note that the word appears at the end of the first line of Ashrei (coming from Psalm 84:5). Perhaps the editors put that line there just to remind benei mitzvah kids to hush and let the congregation respond with the subsequent line?
Whether a note for pausing or for singing higher, or an emphasizing adverbial interjection, selah continues along its way. Sometimes it even finds itself in modern songs and movies, even in the writing of one of our heroes, Robert Benchley.
Your correspondent, not knowing enough, surmises (and educes) that since the Masoretes decided to put a dagesh hazak into the samekh when “selah” falls after an open vowel, they must have been pronouncing the word at that time (between the fifth and tenth centuries CE), rather than simply obeying it. Its prior history remains an enigma.
Thus we may well be unwittingly reading, chanting, and even emphasizing mere stage directions aloud. Such a production. Exīmus, bimah left. Selah.