Found this on Wikipedia today. Pretty interesting.
In computing, the term wheel refers to a user account with a wheel bit, a system setting that provides additional special system privileges that empower a user to execute restricted commands that ordinary user accounts cannot access. The term is derived from the slang phrase big wheel, referring to a person with great power or influence. It was first used in this context with regard to the TENEXoperating system, later distributed under the name TOPS-20 in the 1960s and early 1970s.
The term was adopted by Unix users in the 1980s, due to the movement of operating system developers and users from TENEX/TOPS-20 to Unix. Modern Unix implementations generally include a security protocol that requires a user to be a member of the wheeluser privileges group in order to gain superuser access to a machine by using the su command.