Today's Parola del Giorno is the expression, culo e camicia (literally, it means bum and shirt; read more about the expression from the Italian version of Wikipedia), which means thick as thieves or hand in glove.
It is used to describe a really close connection/rapport between two people, such as two close friends.
I wouldn't use this expression in a business meeting, but among friends and family it should be permissible:
Con un mio collega ormai siamo diventati culo e camicia, praticamente stiamo assieme 19 ore al giorno per via del lavoro e andiamo pure a fare sport assieme. Siamo molto affiatati.
With a colleague of mine we have now become thick as thieves, and we are practically together 19 hours a day through work, and we even do sport together. We are very close.