A Fortnight is an old English term for 14 days. A uFortnight is therefore a micro (1/1000000) of 14 days.
14 days * 24 hours per day * 60 minutes per hour * 60 seconds per minute = 1,209,600 seconds 1,209,600 / 1000000 = 1.2096 seconds.
OpenVMS uses this as the measurement of units for the system parameter TIMEPROMPTWAIT. However, it implements a very rough approximation of a uFortnight by treating it as a second. It has been said that this was put in as a joke and ended up being released in production.