From petra (rock). Rocky, consisting of rock with thin soil over it. Used in the Parable of the Sower for seed that falls on rocky ground.
In Jesus' parable, seed on rocky ground sprang up quickly but had no root and withered when the sun came up (Mark 4:5-6). This represents those who receive the word with joy but have no depth — when trouble comes, they fall away. Quick growth without deep roots produces impressive but temporary faith.