The success of his first restaurant was just 'beginner's luck'. [=he succeeded because he was lucky, as beginners sometimes are]
a swimming class for complete 'beginners' = a 'beginner' class
(..) They're in the trees up ahead. You take the trees, I'll take the bushes. Will you two beginners cut it out? Well, we're just trying to spot an ambush. Morons. I've got morons (..)1969 Butch Cassidy and the Sundance Kid