What is the origin of 'common or garden'?

The derivation of the phrase obviously does have something to do with gardening, or more precisely, agriculture. Its original meaning, as has already been said, relates to the type of plant, fruit or vegetable which is found frequently in gardens or on "commons". (Historically, "commons" were the large patches of grass or woodland that ancient rural villages designated as being for the use of the community as a whole.) If such a plant is found growing in "the common or garden" it is likely to be unexceptional because of its abundance. The phrase has since come to be applied to anything that is common or unexceptional. (I was going to say "run of the mill" but that would be opening a whole new bag of worms, to coin a phrase).

I always assumed it came from the way we describe species of animals or plants. We call lots of abundant species the 'common ____', just a quick google search throws up the common shrew, the common vole, the common pheasant, to dissociate them from the less common species such as the water vole, the pygmy shrew and the golden pheasant.

The term garden is used similarly in colloquial language, especially for plats. As someone said earlier you can call a common rose a garden rose to distinguish it from a highly cultivated and rare species more likely to be found in some botanical garden.

So the term 'common or garden _____' just comes from these phrases used about animals and plants, and was just applied humorously to other objects, such as a common or garden bicycle to distinguish it from a racing bike or a BMX.

It means that it is no frills and generally unremarkable, in that it is easy to come by and not in any way fancy.

Think of the roses you may grow in your backyard- they are pretty common and wouldn't likely win a gardening competition; they simply aren't special. That said, if you were to mention roses, most everyone would be familiar enough with the flower to know what you are talking about. You would have yourself some garden variety roses.