To put "shit on toast" is an expression of how awesome something is. When you really like something - a song, for example - it is your "jam." And what do we do with jam? We put it on toast. As a result, anything awesome is shit put on toast.
Did you see that one-armed clown riding the unicycle over that pile of puppies?
To put "shit on toast" is an expression used to describe nothing or nothing desirable. Similar to jack shit or if there was indeed a culinary dish with shit or faeces on warm breed it would be considered undesirable by most.
I went to the bathroom and man with no regards to cleanliness, walked passed me without washing his hands.. He left me shit shanked and toast I thought to myself as I grabbed the door handle...I'm not hungry any more..