Something that comes out of left field is unexpected, surprising, or unusual. Often, it seems to have little connection to what people were discussing or expecting. The expression is informal and especially associated with American English.
The baseball connection
The expression is generally traced to American baseball. Left field is the area of the outfield to the left when viewed from home plate, where the batter stands. However, the exact reason this particular position became linked with unexpected or unusual things is uncertain. The baseball background is useful context, but a neat story explaining every part of the expression would suggest more certainty than the evidence supports.
How people use it today
You can use the expression for a sudden announcement, an unexpected question, or an idea that seems unrelated to the conversation. It does not automatically mean that something is bad or foolish: an unexpected suggestion might be excellent.
- Her decision to sell the café came out of left field. Nobody expected her announcement.
- His question about dinosaurs came out of left field during our budget meeting. The question seemed unrelated to the discussion.
For a straightforward alternative, use unexpected or surprising. In a formal report, “The announcement was unexpected” will usually sound more neutral than the baseball expression.