So when browsing Unicode characters, I stumbled upon one mysterious case, esh.
The upper case Ʃ looks very similar to sigma Σ which is used for summation notation ∑.
The lower case ʃ is also suspiciously similar in appearance to the integral ∫ which is also used for sum-related things.
I am fairly confident that, since it is a latin character, the connection to sigma is not a coincidence, but did the person who first devised the integral syntax know of the existence of this character, and decide to use a tall s or is it a coincidence because both just happen to be tall s's; one as a lowercase letter the other as a symbol intended to allude to the first letter of the word which brings to mind the symbol which represents it infinitely recursively forever? which it represents?
For the summation symbol $\Sigma$, the use of the capital Greek letter for "s" is due to the use of Latin in Early Modern Europe as universal language; see summa : (mathematics) sum, summary, total.
The first usage is due to Leonhard Euler in 1755; see Institutiones calculi differentialis, page 23 :
Similarly for the long $s$ :