The ancient Greek word for "fish": Ίχθυς (ichthys, ichthus, ikhthys) appears in classical texts and the New Testament. Early Christians often referenced Matthew 4:19 “Follow me, and I will make you fishers of men.” when explaining the IChThUS symbol.

The IChThUS Acronym
The word ἰχθύς became a powerful acronym in early Christianity:

Greek Letter	Word	English Meaning
I (Iota)	Ἰησοῦς	Jesus
X (Chi)	Χριστός	Christ
Θ (Theta)	Θεοῦ	of God
Y (Upsilon)	Υἱός	Son
Σ (Sigma)	Σωτήρ	Savior

IChThUS = Jesus Christ, Son of God, Savior

Secret Symbolism: During persecution, early Christians used the fish symbol as a covert identifier. One believer would draw a crescent in the sand. If the other completed the shape into a fish (the IChThUS), it signaled shared faith.

*Note: Modern Greek uses Ψάρι (psari) for "fish," but this is not used in early Christian symbolism.