Spanking \Spank"ing\, a.1. Moving with a quick, lively pace, or capable of so doing; dashing.
Four spanking grays ready harnessed. --G. Colman, the Younger.
2. Large; considerable. [Colloq.]
Source: Webster's Revised Unabridged Dictionary (1913)
Spank \Spank\, v. t. [imp. & p. p. Spanked; 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.