The Greek adjective aperitmētos (from a- 'without' + peritemnō 'to circumcise') means uncircumcised — lacking the covenant mark of circumcision. In Stephen's speech in Acts 7:51, it is used metaphorically of spiritual hardness.
Stephen's charge to the Sanhedrin that they were 'uncircumcised in heart and ears' (aperitmētos) draws on a long prophetic tradition (Jeremiah 4:4; Ezekiel 44:9) that distinguished outward circumcision from inner covenant faithfulness. Having the physical mark while resisting the Spirit's work made them, in God's economy, as uncircumcised as any pagan. Paul develops this theme extensively in Romans 2:25-29 and Philippians 3:3: true circumcision is of the heart — the Spirit's cutting away of the flesh's dominance. The aperitmētos charge was devastating to its recipients because it denied their most fundamental religious identity — and redirected that identity to the inner life.