Here the word is ordinary and the date of origin is surprising. The OED dates the adjective to the beginning of the 15th century. Anybody who wants to know what pithy means need only think of the core of a stem.
Pith itself is an Old English word (peoþa or piða).
My favorite spelling and example of the adjective is from 1529. Little bit of python in there.
T. More Supplyc. Soulys i. f. xv, The sore pyththy poynt where wyth he knytteth vppe all hys heuy matter.