The Greek noun askos (ἀσκός) means "wineskin, leather bottle" — a vessel made from the skin of an animal, used to hold wine or water. Wineskins appear in one of Jesus's most important parables about the incompatibility of the old covenant order with the new.
In Matthew 9:17, Mark 2:22, and Luke 5:37–38, Jesus says: "No one pours new wine into old wineskins. If he does, the new wine will burst the skins, the wine will run out and the wineskins will be ruined. No, new wine must be poured into new wineskins." The parable, set in the context of questions about fasting, teaches that the kingdom Jesus inaugurates cannot be contained within the old structures of Judaism. The Mosaic law, temple system, and fasting practices were the old wineskins — good in their time, but insufficient containers for the new covenant reality. The Holy Spirit is the new wine; the church in Christ is the new wineskin. This is one of Scripture's clearest teachings on the radical newness of the New Covenant.