a worthless lazy fellow
"After working overtime for weeks, I was furious to see my manager let that loon off early again."
a person with confused ideas; incapable of serious thought
"After months of trying to convince everyone that aliens built our local pizza shop, I finally had to admit he was just a loon."
An idler, a lout.
"The sudden yodeling cry from overhead made us all jump as a loon dove into the cold lake to catch a fish."
Any of various birds, of the order Gaviiformes, of North America and Europe that dive for fish and have a short tail, webbed feet and a yodeling cry.