1. To misplace or mislay someone or something in some particular location. In this usage, a noun or pronoun is used between "lose" and "in." I lost my phone in the airport somewhere. My parents actually lost me in a supermarket when I was two.
2. To become so mentally involved in or preoccupied by something as to be oblivious to everything and everyone else around oneself. In this usage, a reflexive pronoun is used between "lose" and "in." I love losing myself in a good book. He had lost himself in the stunning scenery, so he hadn't heard what I said.
lose something in something
to misplace something in something. I lost my wallet in the barn. Did someone lose something in the dining room?
lose oneself in someone or something
to be thoroughly absorbed in someone or something; to become engrossed in someone or something. Frank loses himself in his children when he is at home. When I lose myself in my work, time just rushes by.
*lost in something
enveloped in something; engrossed in something. (*Typically: be ~; get ~.) Ed sat under the tree, lost in reverie. Excuse me. I didn't hear you. I was lost in my own thoughts.
See also: lost
lose oneself in
Become deeply absorbed or involved in, as in Doctors are notorious for losing themselves in their work. This expression alludes to becoming so absorbed as to forget oneself. [c. 1600]
To cause someone, especially oneself, to become so mentally involved in something as to lose all awareness of everything else: After a hard day at work, I went home and lost myself in a book. I tried to ask her a question, but she was lost in thought.