plank down vs drop

plank down is not a noun while drop is a noun.

Word	Noun	Adjective	Verb	Adverb
plank down	No	No	Yes	No
drop	Yes	No	Yes	No

As verbs, drop is a hypernym of plank down; that is, drop is a word with a broader meaning than plank down:

plank down (noun)	drop (noun)
	the act of dropping something
	a central depository where things can be left or picked up
	a curtain that can be lowered and raised onto a stage from the flies; often used as background scenery
	a sudden sharp decrease in some quantity
	a free and rapid descent by the force of gravity
	a predetermined hiding place for the deposit and distribution of illicit goods (such as drugs or stolen property)
	a steep high face of rock
	a small indefinite quantity (especially of a liquid)
	a shape that is spherical and small

plank down (verb)	drop (verb)
drop heavily	give birth; used for animals
	grow progressively worse
	go down in value
	stop pursuing or acting
	change from one level to another
	lower the pitch of (musical notes)
	fall or sink into a state of exhaustion or death
	leave undone or leave out
	omit (a letter or syllable) in speaking or writing
	utter with seeming casualness
	lose (a game)
	take (a drug, especially LSD), by mouth
	remove (cargo, people, etc.) from and leave
	to remove
	cause to fall by or as if by delivering a blow
	let or cause to fall in drops
	to fall vertically
	let fall to the ground
	fall or descend to a lower place or level
	pay out
	stop associating with
	terminate an association with
	hang loosely