The Greek adjective asteriktos means unstable or unsteady — lacking the firm foundation that comes from being established in truth. Peter uses it in 2 Peter 2:14 (false teachers ensnare the unstable) and 2 Peter 3:16 (unstable people distort Scripture to their own destruction).
Peter's warning about asteriktos souls is urgent pastoral theology. False teachers deliberately target those unestablished in the faith — the young, the doubting, the spiritually rootless. The antidote is being stablished (sterizo) in the truth. Peter writes specifically to 'stimulate you to wholesome thinking' (2 Peter 3:1) and 'establish you in the truth you now have' (2 Peter 1:12). Spiritual stability comes through the Word, community, suffering rightly received, and the Holy Spirit. An unstable soul is not condemned — it is in danger, and the Gospel is the anchor.