The Greek adverb bareōs means 'heavily,' 'dully,' or 'with difficulty' — particularly used of dull, sluggish hearing. It appears twice in the New Testament (Matthew 13:15 and Acts 28:27), both times quoting Isaiah 6:9-10 in the phrase 'their ears are dull of hearing' (tois ōsin bareōs ekousan). The word describes a spiritual heaviness that blocks the reception of truth.
The Isaiah 6 quotation that Jesus applies to His generation, and Paul applies to Israel in Rome, is one of the most sobering passages about spiritual hardening in Scripture. Bareōs — dull of hearing — describes a heart that has been so saturated with rejection of God's word that it can no longer easily receive it. Yet even in this context, the quotation ends with the hope: 'lest they should... be converted, and I should heal them' — the possibility of restoration remains open to those who turn.