things that's not nailed down

[IQU3838]

Hi,

Could you help me with the underlined part?
THank you.

A: I feed your dog blueberries, and he likes them. Do you know he likes fruit?

B: He'll eat anything that's not nailed down.

I know what "nail down " means when it is used as verb, but I don't know its usage in this context.

Please help me with this phrase, thank you.

 

[Barb_D]

If something is nailed down, you can't pick it up.

It's a somewhat common idiom (anything/everything that's not nailed down) to mean "just about everything." It's similar to "everything but the kitchen sink."