It is very interesting that the symbol for Psychology has different stories about its real origin.
Some would say: "It is the Greek letter psi which is the first letter of the Greek word psuchê, which is the one of the roots of the term "psychology."
As can be seen from the image below, the symbol for psychology represents the penultimate letter of the Greek alphabet, psi, which is also the first letter of the Greek word psuche, meaning mind or soul, from which the term psyche was derived.
According to some literature, "In ancient times, anything that had to do with mental illness was categorized as diabolic. Since they thought that all mental illnesses and abnormal behavior was caused by the devil and evil spirits. Ergo, the people who studied this abnormal behavior ("psychologists") were also thought of as evil. Because the devil holds this fork-like figure in his hand, psychology (back then considered a diabolic branch of study) took this symbol as their own.
As an irreverent display of defiance, psychologists adopted the devil fork shape as their official symbol.
However, some literature would not accept the devil's fork theory on how Psychology got its symbol. They contend that in "ancient times" (i.e., before Christ) there was no concept equivalent to the Christian "devil." The Greek and Roman gods were neither totally good nor all evil. The "gods" acted like humans with emotions and round character who knows how to love, get angry, plan schemes, and feel sympathy and the like. There being no devil, the "fork-like" figure did not have the same significance for them that it does for many of us now. The trident (a fish spear) was, far from being associated with evil. The trident is the symbol of the god of the sea, Poseidon (for Greeks), or Neptune (for the Romans).