to be thirsty: to feel the body's craving for water, and by extension to long for something as keenly as a parched person longs to drink
At its root this word names one of the body's plainest and most urgent needs: the craving for water. In the dry lands of the Bible thirst was a daily companion and, on a long road or in a siege, a real threat to life. That physical force is exactly what gives the word its power when it is turned toward the soul.
In its plain sense it describes ordinary thirst. Jesus says from the cross, "I thirst" (John 19:28); the King in the parable of the sheep and the goats was thirsty and was, or was not, given a drink (Matthew 25:35, 25:42); Paul writes that the apostles went hungry and thirsty (1 Corinthians 4:11). Alongside it runs its natural partner, πεινάω, the craving for food, and the two are almost always voiced together as one picture of want.
The Gospels and Revelation then reach for the same word to name a longing of the whole person. To "hunger and thirst for righteousness" (Matthew 5:6) is to want it with a body's urgency, not a mild preference. Jesus promises that whoever comes to him and believes "will never thirst" (John 6:35), and Revelation offers the thirsty the water of life freely (Revelation 21:6; 22:17). The figure works only because the literal thirst behind it is so real.