There are lots of spelling variants on the hickory-dickory-dock theme: hickere-dickere-dock, hickety-dickety-dock, and so on. What they all have in common is obscure origin. Peter Opie and Iona Opie, in The Oxford Dictionary of Nursery Rhymes, suggest the meaning of of the words lies in sheep-counting jargon from northern England.

According to this Wikipedia link, such counting-words for eight-nine-ten in Borrowdale, Westmorland, or Derbyshire Dales, are hovera-dovera-dick with variants (many similar, some not) in other districts. It’s not much of a stretch to see these numbers evolving into the nursery rhyme:

Hickere, Dickere Dock,

A Mouse ran up the Clock,

The Clock Struck One,

The Mouse fell down,

And Hickere Dickere Dock.

[from William S. Baring-Gould and Ceil Naring-Gould: The Annotated Mother Goose]

The rhyme describes simply an abridged version of the 360º journey of the hour-hand around a clock. As such, it might be noted that this is probably the earliest use of the word mouse to mean a “pointing device”.