Largest college fraternity in the U.S., familiarly

SIGEP

Think of the Greek letters that sound like a short way to say "significant" plus "epsilon."

This clue asks for the familiar nickname of the largest college fraternity in the U.S., which is Sigma Phi Epsilon—commonly shortened to "SigEp" (pronounced "sig-ep"). The answer is written as "SIGEP" to fit crossword style, dropping the capital "E" for brevity. The fraternity is often called this in casual conversation, making it a well-known shorthand among Greek life circles.