Coined by GeraldWeinberg: 

A lullaby word "lulls your mind into a false sense of security" yet remains ambiguous enough to allow for the opposite interpretation

Examples:

should ("The customer should provide the sales data by August 1" -- does this mean "must" or "might"?)

just ("We'll just need to install Oracle" -- yeah, right, after just a few weeks of sweat and pain)

soon

very

and my personal addition to the list:

trivial (use 'straightforward' instead)

-AlexChaffee