<p>Dictionary.com is a horrible, horrible dictionary quite frequently.</p>
<p>that being said here it does match the OED :)
[quote]
Of, pertaining to, or characteristic of a pupil at a preparatory school (see PREPARATORY a. 2a); immature; (see also quot. 1980). Also as n., a pupil at a preparatory school. Hence preppiness. [OED]
[/quote]
</p>
<p>However it does have a more complex common usage, I think...</p>