How Well Do You Know English Slang?
1660s, "very big or fine," later (especially of horses) "moving at a lively pace" (1738), perhaps from a Scandinavian source (cf. Danish spanke "to strut").
1727, possibly imitative of the sound of spanking. Related: Spanked; spanking. The noun is from 1785.
Very; extremely, esp in an admirable sense
[1666+; origin uncertain]