My snarky response is always:
It means she didn't know what she was doingwhat could this symbol mean?
I'm saying this because if I make a quip like that, then usually someone will come out of the woodwork with a "Oh, this is the symbol use in the bladebla region of bladenstein during the hungoverian period of 1892-1897" to make me look like a fool!!!
But this goes back to my post of "composer as infallible" - some of these people were working in relative isolation and developing their own idiosyncratic notation (without explaining it...) and some of them just "did what they thought was right" when it wasn't. I'm not saying that's surely the case, but it is a possibility. And considering the colon after the dynamic...maybe she only had access to really old scores or something - who knows.
Looks like a tongue...