The fifteenth letter of the Hebrew alphabet is samech (or samekh). There is some debate as to the origin of the name. Klein says it comes from the word סמך and means "support, fulcrum". E-hebrew suggests "spine" from the same Hebrew root, David Sacks suggests "pillar", while others say that root meant "peg, spike". Another theory is that the letter is similar to the shape of a fish, and therefore it is related to the Arabic samak, meaning "fish" (the Hebrew word for trout שמך - shemech - is related to this Arabic word.)
Another confusing aspect of samech is which Greek and Latin letters came from it. Sacks writes that for the letter sigma the Greeks took the sound of "s" from samech and the Greek styling of the name, but the shape and placement of the letter in the alphabet (#21) was borrowed from shin. From sigma, we get the Latin letter "S". On the other hand, some theories claim that the Greek letter chi, which led to the Latin letter "X" came from the shape of the Hebrew samech. As the shape of the letters evolved in each alphabet, we have the Hebrew version currently looking like a circle, and the Latin version as X. Kind of like tic-tac-toe, no?
The verb סמך has a number of meanings: "to support, sustain, uphold", "to lay (hands on), lean", "to draw near, approach". From the sense of "lay hands on" we get the concept of semicha - סמיכה - Rabbinic ordination, derived from the method of transfer of authority.
The Hebrew word for blanket - שמיכה - semicha, also derives from the root סמך meaning "to support". It appears once in the Tanach - Shoftim 4:18. The commentaries disagree as to the meaning there - some say it was a kind of blanket, others an article of clothing. Stahl says that it might have been so called because the garment was thick, and therefore is related to the Hebrew word for "dense, thick" - סמיך samich. This also goes back to סמך - something dense is pressed on, drawn close together.
Samech alternates with sin, particularly in Aramaic (כנס כנש), as well as with zayin and tzade ( אסר אזר אצר ). Steinberg claims it can also change with tav - as in תמך סמך .