What does "hot diggity dog" mean?

The exclamation "Hot diggity!". It's clearly an excited interjection of approbation, and I understand the "hot" part of it, but where does "diggity" come from?

The full expression is "hot diggety dog," which is from "hot dog!" That simpler form, pronounced with equal accents on both syllables, was used as an exlamation of surprise and delight in the U.S. in the late 19th century. Pronounced that way, it implies neither a frankfurter nor showing off in some dangerous sport.

The Oxford English Dictionary defines "hot dog" (the interjection) thus:

"Chiefly U.S. slang. Expressing delight or strong approval. Cf. HOT DIGGETY DOG int.
[1893 Recensio (Miami Univ., Oxford, Ohio) I. 92 Favorite Ejaculations... 'By Joe!'..'Hot! Dog!'..'Great Scot, man!']", , ,

Other examples follow, and I have often heard the expression used in this way. "Hot diggety dog" is an example of tmesis, meaning that another word has been inserted into an expression. "Diggety" is not exactly an independent word; it is just a way of protracting the expression for emphasis.

The OED defines "hot diggety dog" this way.

"An exclamation of joy or surprise. Cf. HOT DOG int.
1923 T. A. DORGAN in L. Zwilling TAD Lexicon 46 Hot diggerty dog. 1927 Sun (Baltimore) 3 Apr. II. I. 12/2 When it comes to 'hot dog', there's no more to be said, unless it is, perhaps, to add a frill and make it 'hot diggety dog'."

"Hot diggity" without the dog appears as early as 1924

The OED explains "diggety" thus:

". . . with alteration of the vowel of the second syllable and suffixation (compare -ETY suffix)." For another example of the -ety suffix, the OED mentions lickety split.