The professor mentioned Plato, apropos of the philosophical debate we were having. The preposition 'apropos' has a fascinating etymology rooted in the French language. It is a fusion of the words 'à' (meaning 'to') and 'propos' (meaning 'purpose' or 'topic').

Apropos typically functions as an adjective describing what is suitable or appropriate (“an apropos comment”), or as a preposition (with or without of) meaning “with regard to,” as in “apropos (of) the decision, implementation will take some time.” The phrase “apropos of nothing” suggests that something does not relate to a specified topic.

According to Collins English Dictionary of the English Language, the word apropos can be used as an adjective, adverb or preposition. The word apropos describes something that is related to the subject or topic at hand, or appropriate for a certain situation.

It is a fusion of the words 'à' (meaning 'to') and 'propos' (meaning 'purpose' or 'topic'). This etymology beautifully encapsulates the essence of the word, as 'apropos' is used to indicate relevance or appropriateness in relation to a particular topic, situation, or context.