spank · verb, noun1
Uncertain. Possibly imitative, or perhaps a special use of Etymology 2 (below) in the sense of a horse stamping its foot against the ground. There is no evidence of a relation with Portuguese espancar (“to batter, strike”).
spank · verb, noun2
Uncertain. Likely of Dutch or Low German origin. Compare North Frisian spanke (“to step with pride”), Saterland Frisian spankje (“to splash”), dialectal Dutch spanken (“to limp, hobble”), Dutch spankeren (“to run away quickly, gallop off”), German Low German spenkern (“to burst, explode, shatter, splash, spray”), Danish spanke (“to strut”), Swedish spånka (“to strut proudly”); originally, equivalent to span + -k. Compare also dialectal English spang (“to leap, bound; shoot out, fling”).
Etymology from Wiktionary, CC BY-SA 4.0.