I get Wikipedia as the top result. Maybe Google is making a guess about your preference based on your history? Try the search in incognito/private mode and see.
So maybe it's based on location? Ranked higher based on browser, IP, or something else?
On a desktop, I see what you mean; UrbanDictionary is indeed at the top of the plain-text search results.
Not going to defend Google at all. But for myself, if I'm looking for a Wikipedia-grade introduction to some topic, I usually just search Wikipedia directly. Wikipedia has become so ridiculously useful that it is the top result of a Google search half the time anyway. Wikipedia is a better human-curated web index than any of the old-school 1990s human-curated web indexes ever were.