apropos of nothing

"The Briefcase" の主人公、ツキコとセンセイは相当な飲兵衛です。
Sensei lingered over his bottle of sake. Whenever the Giants' pitcher struck out a batter, or a Giants player got a hit, he nodded vigorously.
"What's the matter, Tsukiko?" Sensei asked me after a home run at the top of the seventh inning gave the Giants a three-point lead over the Tiger.
"I'm just tapping my foot."
I had been nervously shaking my foot since the Giants had gained their lead.
"The nights are getting cold," I said, apropos of nothing and not even in Sensei's direction, but more toward the ceiling. At that moment, the player for the Giants got another hit.
"apropos of nothing" はどんな意味でしょうか？　辞書を見ます。"apropos of nothing" の項があったのはWiktionary.comだけで、その説明は次ぎのようでした。
1.Without reference to anything. 2.Without any apparent reason or purpose.
"apropos of" の見出しを載せている辞書は幾つもありました。
・Collins Dictionary: with regard to; in respect of: He is currently contesting his mother's will, he blurts out, apropos of nothing.
・Merriam-Webster's Online Dictionary: with regard to : concerning: apropos of our earlier conversation, here's that file I mentioned
"apropos of nothing" は「唐突に」、「突然」とか「何の関連もなく」という感じでしょうか。