It's a reference to chopping down trees. The 'it' is a tree. You cut the trunk with an axe, then as the tree begins to fall, you shout 'timber' to warn people to get out of the way.

Apparently that means to get wired in the sense of let's party, drink, and dance, but not smoke weed. Or so that is what urbandictionary.com states in reference to the song. I guess the comparison is between yelling timber when a tree goes down (that is "falls to the ground") and yelling or referring to a party as timber when it (that is, the party and the fun) is going down / happening. Hehehe :)

Oh, are you referring to lyrics from Pitbull's song Timber?

By the way, these are slang expression and not standard English. You can always look up their meanings on http://www.urbandictionary.com though. That website is a place where native speakers record the meanings of slang phrases which aren't officially in the dictionary. Just be warned though that a lot of dirty and naughty words and phrases appear there too. Hehehe.

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