Here's what it *could* refer to:
* A lamb born in a specific year: This could be a way to identify a lamb based on its birth year, similar to how we refer to "yearling" horses.
* A lamb that's a year old: This is more likely a reference to the lamb's age, rather than a specific type of lamb.
* A lamb that's part of an annual production cycle: Some farms might have a specific time of year when they breed their ewes and produce lambs, and "yearly lamb" could refer to a lamb born within that cycle.
To understand what "yearly lamb" means, you need more information, such as:
* The context in which you heard the term: Where did you hear it, and what was the conversation about?
* The speaker's background: Do they work with sheep, or are they just using a casual term?
If you can provide more context, I might be able to help you figure out what "yearly lamb" actually refers to.