Origins
Target is a word that turned itself inside out. It began as protection and ended as vulnerability. The original target was a small round shield — from Old French targette, diminutive of targe, from Frankish *targa meaning 'shield'.
Medieval archers needed marks to shoot at during practice, and small round shields made convenient ones. Set a targe on a post, step back, and shoot. Over time the word migrated from the shield to its new role. By the 16th century, a target was no longer something you stood behind but something you aimed at.
This is one of the most complete semantic reversals in English. A word meaning 'defence' now means 'point of attack'. The shield became the bullseye.
Proto-Indo-European Roots
The Proto-Germanic root *targō meant 'edge' or 'border' — the rim of a shield, the frame that holds it together. German preserves this in Zarge, meaning 'frame' or 'border'. Italian targa means 'plate' or 'plaque', and gave its name to the Targa Florio — a Sicilian road race whose trophy was a shield-shaped plate.
Modern English has extended the word far beyond archery. Sales targets, military targets, target audiences — all preserve the idea of aiming at a fixed point. None remember the shield.