Spell-it-how-it-sounds version of "Psych."
"Sike" (or, more properly, "Psych") is a declaration that you just deliberately messed with someone's mind. Literally, "I fooled your
brain."
"Psych" is a Greek root-word meaning "mind" - found in many other
words, such as psychiatry, psychotic, psychic, psychedelic, etc., all of which relate to the mind. Note that the first syllable of all those
words sounds like "sike."
The "Sike" spelling is most often used by
people who have never read a book and are therefore only familiar with hearing the language spoken out loud rather than seeing it in written form.