Boyd’s Bible Dictionary:
(gad, strike). A rod spiked at the end for driving oxen (Judg. 3:31); and iron-shod at the other end for cleaning plows, or even for plowing (1 Sam. 13:21).
Concise Bible Dictionary:
A long slender pole with a sharp point at one end, used for guiding and urging the oxen in plowing. It can also be used as a formidable weapon. Shamgar slew six hundred men with an ox goad (Judg. 3:31; 1 Sam. 13:21). It is applied metaphorically to the energy imparted by the words of the wise (Eccl. 12:11). The goad is alluded to in Acts 9:5; Acts 26:14, translated “pricks:” if the ox kicked against the goad, he only hurt himself the more: as do all those who oppose God.