What does 「ものさ」 mean here? In Miffy - ! at around 5:14 Miffy with her family planned to go to the beach and before they started play, Uncle said: **** Weblio says: `` But it's not used by women or children here, but by uncle, what does mean here?

Unfortunately you are looking at a wrong definition.

I suppose you are referring to this one under the (final particle) section in the entry:

> ****

But the correct one is under (noun) which is way below:

> ****
> ⋮
> …

So the basic construction is the following.

>
> _I used to build castles for fun._

And it seems to be rarely taught, but there are final particles that hides the copula and those not. is the former, that's why you don't see in that sentence.