Nir (H5214) means to till or break up unplowed ground. It is a strong agricultural image of preparing hardened soil for planting. The noun form (nir) refers to fallow ground — land that has lain unworked and hardened.
Hosea 10:12 gives this word its most famous theological application: 'Break up your unplowed ground; for it is time to seek the LORD.' Nir becomes a metaphor for repentance — the interior work of breaking up hard, resistant hearts to receive the seed of God's Word. Jeremiah 4:3 uses the same image. The spiritual lesson: before sowing righteousness, the hardened soil of the heart must be broken up. This is the work of the Holy Spirit and genuine repentance.
Nir sits within the agricultural vocabulary of the OT that includes sowing (zara), reaping (qatsar), and harvest (qatsir). The fallow ground metaphor is one of the most potent in the prophets for describing spiritual hardness. Jesus' Parable of the Sower (Matt 13) works within this same agricultural framework.