Need

Here's a simpler word that has also aquired a new meaning that isn't in dictionaries.
We all know what 'need' means, it expresses a requirement for something. Thus "I need a drink" indicates that the speaker is thirsty.

However consider these:

"You need to read this book"
"What you need to understand is that ..."
"You need to be quiet folks"
- which is a less aggressive alternative for the command "Be Quiet!"

The emphasis here has shifted to the speaker telling others what the speaker thinks they need. The shift in meaning has in some cases reached the point where in context the other person plainly doesn't want to comply:
"You need to give me all your money right now!"

Smirking

Chatting on the Internet gets one close and personal with language as she is spontaneously typed, including an often ignored factor - the differences between dictionaries and how we actually use words. Here's an example I finally got puzzled enough to look up just now, "smirk".
Online dictionaries agree about this word:
Verb: Smile in an irritatingly smug, conceited, or silly way.
Noun: A smug, conceited, or silly smile: "a self-satisfied smirk".

However when 'real people' are asked to define it the negative content mostly isn't there:
http://answers.yahoo.com/question/index?qid=20090413212934AAMHIBK
"Smiling very slightly, like you are trying not to. Usually one side of your mouth is smiling more. And it usually means that that person is up to something"

Americans use this word regularly, Brits do not, and dictionaries have yet to notice that its meaning has changed.