the word spread refers to the widening of the hips and ass. this usually happens to secretaries, who sit in a chair all day snacking on sweets; thus the name, secretary spread.
"Dayumm, that secretary has a wide ass!" or "I feel like I'm getting secretary's spread from sitting all day."
Tendency of (mainly female) clerical and secretarial staff in large organizations to engage in office romance/sexual activity with superiors as means of professional advancement.
"Women have such low self-esteem that sometimes the secretary spread seems like the only route to success." New York Post