So I looked it up. Google is always a good friend when I'm curious about something.
According to Dictionary.com, Ides comes from the Latin word idus, roughly marking the 15th day of March, July & October and the 13th day of the other months of the year. It's the halfway point of each month.
The Ides of March is a metaphor for impending doom. Julius Caeser ignored the warning "Beware the Ides of March" and was stabbed in the back on March 15, 44 BC - also giving us the term backstabber.
This is just my summarized version, use the link above to read the full explanation.