What does sing like a lark mean?

Answer 1

To "sing like a Lark" has multiple meanings in modern usage:

It can refer to the beauty or care-free nature of one's singing.

However, in criminal vernacular to "sing like a lark" can mean to turn over documents or information to another party, without the knowledge or consent of the holder.

Answer 2

a lark or skylark is a bird that is known for it's distinctive bird song.