In this situation, it's a shortened version of the expression "getting ploughed under"
In brief, since this story is about skateboarding, it's someone saying "get out of the way or you're going to get knocked to the ground"
A plough is a farm implement (tool) that turned the earth over, before the farmer plants his fields.
So, a plough turned the earth (dirt) under.
In slang, ploughed is also used to discribe being REALLY drunk with alcohol. As in "Wow, Jim was really ploughed at the party last night".
Imagine getting run over by this plough, and you'll get the meaning.
It isn't really a rude word, as in profanity.
I wouldn't say it to someone I respect though.