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]