For this proverb see Diogenianus 7.55 ("In reference to those conquering everything; for with such a spear Procris used to conquer everything.") and Eustathius, Commentary on the Odyssey 1.420.41-43 ("In reference to those throwing a spear accurately and not missing;" cf. the proverb at eta 286), where the noun is also in the accusative case. This was in mythology, however, an ill-omened spear and can hardly have been used as a proverb with positive implications -– unless all memory of the striking myth had been lost.
Procris is one of the six Athenian "Maidens" (*pa/rqenoi, pi 668, cf. OCD(4) 1215), the daughters of Erechtheus. Her marriage to Cephalus, a hunter like herself (kappa 1453, OCD(4) 299), with its pattern of marital jealousy, betrayal and jumping to false conclusions (cf. Proust's méprise de prémisses), constitutes a myth with countless variations. Attempts to reconcile these variants into a single story line have failed both in antiquity and in modern scholarship (e.g. RE and Fontenrose, in bibliography below). The only version well-known today is the delicate, reticent one of Ovid (Metamorphoses 7.690-892, cf. Apollodorus 1.9.4, 2.4.7.2, 3.15.1.2; Hyginus 189; Antoninus Liberalis 41), perhaps echoing the story line of Sophocles' lost play Procris.
The unerring spear (a)/kwn is a light javelin or hunting spear; cf. the a)ko/ntion at alpha 926, alpha 927) is a central element in all versions, for by it Procris met a tragic death from the hand of her husband (cf. tau 429). In one version it was given directly to him by the goddess Eos (Aurora, Dawn), together with the swift hound Lailaps, in an attempt to win his affections away from his wife. But in most versions these gifts were made to Procris in Crete, either by King Minos, whom she healed so that he could have children, or by the goddess of the hunt, Artemis. Ovid passes over the conflicting accounts of just how the deadly spear came to be in Cephalus' possession (most effectively if Eos had stolen it for him) when Procris was spying on him out of unfounded jealousy. Hearing a noise in the bushes he threw it; it could not miss. In some versions he placed it in remorse on her tomb, where it was coveted for its magic powers. According to Hellanicus he was tried by her father Erechtheus at the Areopagus in Athens (FGrH 323a F22) and acquitted. The two received offerings in a hero cult at Thoricus in Attica (Kearns, below, citing Pherecydes FGrH 3 F34), where her tomb may have been.
[1] The Suda has the verb tugxa/nw for aiming and hitting, where Diogenianus uses nika/w for conquering. It is likely that Diogenianus altered the verb he found, for tugxa/nw by definition implies a certain amount of chance in the flight of a missile such as a javelin towards its target (cf. epsilon 3344, eta 286, tau 435, tau 1234, etc.). There was no such element of chance in the flight of Procris' spear, as Ovid points out, "It follows whatever it is aimed at, and fortune does not rule it, once it is thrown" (Met. 7.683).