apropos of nothing

[druiaghtagh]

What does ths mean please anyone?

[THECORBYLOON]

"Apropos" means "with reference to " and comes fae the Fench "a propos" so "apropos nothing" means mentioning something for the first time or going off the subject.

[Clanad]

Not to engage in the picking of nits, I think the meaning is more in line with appropriately... at an opportune time.  It is, as Corbyloon states, from the Old French, through Latin, but an equally valid interpretation would be to the purpose, to the point.  A synonym would be relevant.  It also has a conotation of incidentally.  "Apropos, we were discussing..."

[snowpea]

You cannot define a phrase by breaking down the mechanical parts.  That barely scratched the surface!

Apropos of nothing means more than simply "off topic" or "irrelevant"  Those are merely denotative aspects of the phrase. 

The connotative aspect of the phrase suggests that whatever you are claiming as "apropos of nothing" is not only irrelevant, it is of no value.  It does not edify you in any way and probably makes you a trifle dumber for knowing it. 

See the novel Sir Apropos of Nothing for an interesting read about a character that embodies the multiple dimensions of this singular phrase.

[jno]

it just means 'To change the subject...' because it's apropos nothing that's already been mentioned. But I disagree with snowpea, I don't think it means that what you are about to say is trivial, just that it's not related to what went before.