The Greek adjective apeiros (from a- 'without' + peira 'trial/experience') means inexperienced or unacquainted with something. Its single New Testament use describes spiritual immaturity in Hebrews 5:13.
Hebrews 5:13 uses apeiros to describe the spiritually immature: 'Anyone who lives on milk, being still an infant, is not acquainted with (apeiros) the teaching about righteousness.' The word implies that righteousness is learned through lived experience and practice (hexis), not just intellectual instruction. Spiritual maturity cannot be downloaded — it is forged through the training (gymnasia) of repeatedly choosing right over wrong, walking through trials, and developing discernment. Apeiros is not a permanent condition but a calling to press forward into experience.