Ok, well so you know what the word means and to spare you the effort of bothering to look it up, here is the entry from Chambers
jag1 /jag/
noun
A notch, slash or dag in a garment, etc
A ragged protrusion
A cleft or division (botany)
A prick (Scot)
An inoculation, injection (chiefly Scot informal)
transitive verb (jaggˈing; jagged /jagd/)
To cut unevenly or into notches
To prick or pierce
Now the entry for jab
jab /jab/
transitive verb and intransitive verb
To poke, stab
To strike with a short straight punch
noun
A sudden thrust or stab
A short straight punch
An injection (informal)
ORIGIN: Orig Scot variant of job2
Naomi, which of these is more appropriate for a nurse etc sticking a needle in you?