It is one of the numbers the team has retired, and it was retired for Pat Tillman. As with the other retired numbers, the players had tragic deaths.
Tillman was a seventh round pick by the Cards out of Arizona State and played four years for the Cards, starting 39 games. When free agency came around after the 2001 season, rather than signing a new multi-million dollar contract, he decided to go serve in the military. He was deeply affected by the attacks on September 11.
He was killed tragically in a friendly fire incident in Afghanistan.
Cards fans remember his intensity and the sacrifice he made. He is immortalized outside the stadium with a bronze statue.
Before Tillman, cornerback Robert Massey wore 40 from 1991-1993. He had a pretty memorable game in 1992, returning two interceptions for touchdowns. He made the Pro Bowl that season after intercepting five passes and returning three for touchdowns.
The best player to wear 40 was Bobby Joe Conrad. An 11-year Cardinals veteran. He wore 40 from 1958-1968 in both Chicago and St. Louis. He was a versatile kicker/flanker to start, but stuck to flanker (something like a receiver) after the move to St. Louis.
He was a 1963 All-Pro and a 1964 Pro Bowler. He led the league in receptions with 73 in '63.
Is there anyone else you can remember?