I think I was channelling my dad. He used to say that to Amanda the dog when he gave her a treat, way back when I was but a wee sprat. It's one of those odd expressions that resonate in the back of your head, and all you really remember of the phrase is the phonetic sound of the words and the cadence but not the context or the real meaning.
Tonight I realized the real meaning. It took me a few seconds to mentally shake myself out of it, but I remembered exactly what a tallywhacker was.
I don't remember him saying it to Maggie the dog ten years later, though.