From The Collaborative International Dictionary of English v.0.48:
spanking \spank"ing\, a.
1. Moving with a quick, lively pace, or capable of so doing;
dashing.
[1913 Webster]
Four spanking grays ready harnessed. --G. Colman,
the Younger.
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2. Large; considerable. [Colloq.]
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3. Very; extremely; -- used especially in the phrase spanking
new or brand spanking new, meaning completely new and
unused. [informal]
[PJC]
From The Collaborative International Dictionary of English v.0.48:
Spank \Spank\ (sp[a^][ng]k), v. t. [imp. & p. p. Spanked
(sp[a^][ng]kt); p. pr. & vb. n. Spanking.] [Of unknown
origin; cf. LG. spakken, spenkern, to run and spring about
quickly.]
To strike, as the breech, with the open hand; to slap.
[1913 Webster]