pick at vs trash

pick at is not a noun while trash is a noun.

Word	Noun	Adjective	Verb	Adverb
pick at	No	No	Yes	No
trash	Yes	No	Yes	No

As verbs, trash is a hyponym of pick at; that is, trash is a word with a more specific, narrower meaning than pick at:

pick at (noun)	trash (noun)
	an amphetamine derivative (trade name Methedrine) used in the form of a crystalline hydrochloride; used as a stimulant to the nervous system and as an appetite suppressant
	nonsensical talk or writing
	worthless people
	worthless material that is to be disposed of

pick at (verb)	trash (verb)
express a negative opinion of	express a totally negative opinion of
eat like a bird	dispose of (something useless or old)
pluck or pull at with the fingers