in-the-know

「内情に通じている」。

The New York Timesの記事、'An Italian Beauty Without Foreign Suitors'By INGRID K. WILLIAMS から。

'Not so known outside Italy, that is. Lerici (pronounced LEH-ree-chee) is a jumble of pastel buildings that jockey for attention with its beaches, crescent-shaped coves and rocky cliffs that melt into the sparkling sea. And in July and August, the town is bustling, the beaches filled with local residents, vacationing families from northern Italy and a loyal crowd of in-the-know Milanese'.

ちなみに、jockeyは、この場合動詞として使われていて、 jockey forで、「〜を得るために競争で有利な立場に立とうとする」。bustlingは形容詞で、「にぎやかな」。都市の喧噪などを表現するときにもよく使います。