B.S., profligate is used as a noun - "a spendthrift." (Webster.) You are also wrong in that in the cartoon the word modifies the pronoun in the prepositional phrase.

how can a noun be profligate though? a profligate house? profligate car?

I'm looking at the Sept. 8 strip that highlights the word, "profligate," which is an adjective. The sentence reads "Darn right, It's very profligate of them!"
In that sentence, "Darn right" is in an independent, introductory clause. The subject is "It" (presumably referring to adding new words to the dictionary). The verb is "is." "Very" is an adverb which, in this case, is modifying "profligate." But profligate seems to be acting as the object of the sentence, a role which is reserved for nouns, not adjectives. Adjectives modify nouns, they don't substitute for them. Either that or the author intended to say is profligate, which still would be using it incorrectly (as a verb).

lol