The 'stice' part of the name of the charm is related to modern "stitch" when we refer to a sudden sharp pain in the side. Scholars do not know if there was a particular cause for the pain; rheumatism or lumbago have been suggested, but in my opinion it was more likely to be what it was called: a sudden sharp pain, the kind that seems to have no particular trigger.
The creators of the text guessed at the source of the pain, and claimed their remedy would help in several cases, according to these lines from the charm.
This for you as a remedy for the shot/pain of ēse;
this for you as a remedy for the shot/pain of ælfe,
this for you as a remedy for the shot/pain of hægtessan; I will help you.
ēse refers to the Æsir, the Norse gods; ælfe refers to elves; hægtessan refers to witches. The Wið færstice was supposedly good for pain caused by any of these supernatural sources. Whether it worked for anyone, we'll never know.
Another term for a sudden stabbing pain with no visible source was "elf shot," which I'll talk more about tomorrow.