Wow. Webster's 1828 Dictionary gives: SPANG, n. A spangle or shining ornament; a thin piece of shining metal; something brilliant used as an ornament.

2. An y little thing sparkling and brilliant like pieces of metal; as crystals of ice. For the rich spangles that adorn the sky.

Webster Dictionary, 1913, gives:
Spang (Page: 1377)
Spang (?), v. t. To spangle. [Obs.]

Spang (Page: 1377)
Spang, v. i. To spring; to bound; to leap. [Scot.]

But when they spang o'er reason's fence, We smart for't at our own expense. Ramsay.

Spang (Page: 1377)
Spang, n. A bound or spring. [Scot.] Sir W. Scott.

Spang (Page: 1377)
Spang, n. [AS. spange a clasp or fastening; akin to D. spang, G. spange, OHG. spanga, Icel. spöng a spangle.] A spangle or shining ornament. [Obs.]

With glittering spangs that did like stars appear. Spenser.

Merriam-Webster Online Dictionary (interesting, to be able to follow one source down through time!) has:
Main Entry: spang
Pronunciation: 'spa[ng]
Function: adverb
Etymology: Scots spang to leap, cast, bang
1 : to a complete degree
2 : in an exact or direct manner : SQUARELY

Wordsmyth:
Part of Speech adverb
Pronunciation spaeng
Definition 1. (informal) quickly, squarely, or precisely.
Example He walked spang into the wet cement.

Ultralingua gives what is to me both a better yet controversial explanation:
spang v. < spá[ng] > : 1. To leap. jerk, bang (dialectal); "Bullets spanged into the trees"; SYN: bang. [ETYM: Scot.] I can certainly see where bullets, or 'most anything that ends in s can morph from -s banged into spanged, but what does bang have to do with leap or jerk? That is, a gun will leap and jerk in your hand when you fire it, but in the normal way I think of leap and jerk, I would never associate the word bang with them.

I had been planning to say (before I got all sidetracked) that I've heard this word all my life; dunno if it's Southern, Appalachian, or just old-fashioned; for some reason I am mentally associating it with Mark Twain.