The Greek adverb Hebraïsti (Ἑβραϊστί) means 'in Hebrew' or 'in the Hebrew/Aramaic tongue.' It appears in John 5:2, 19:13, 19:17, 19:20; Revelation 9:11 and 16:16, consistently used to give the Hebrew/Aramaic equivalent of a Greek name or place.
John's Gospel uses Hebraïsti as a literary technique to anchor Greek-speaking readers in the Jewish geographic and cultural reality of Jesus' ministry. When John says the pool is called 'Bethesda' in Hebrew (John 5:2), or that the pavement is called 'Gabbatha' (John 19:13), or that the cross site is 'Golgotha' (John 19:17) — he is insisting that these events are rooted in real history, in real places, in the actual language of Jesus' world. This is not myth. This is the Word made flesh in a specific language, culture, and time. Revelation's use of Hebrew names (Abaddon, Armageddon) in the most cosmic of contexts similarly grounds the final events of history in the covenant history of Israel.