Definition
The apparent widening of the hips and buttocks, as seen when a voluptuous woman sits on a chair.
How People Actually Use It
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.""
Source: Urban Dictionary