From the skull and crossbones my guess and it is only a guess as I have not seen any would be that it is a dagger associated with a WWII Concentration Camp. I could easily be wrong and probably am but I had heard in the past that units operating these camps wore the skull and crossbones.
The "
Totenkopf" (death's head) was worn by two groups in Nazi Germany. The SS and the Army Panzertruppen (tank crews).
The SS included those units that operated the Concentration camps.
The SS skull has the lower jaw, the Army tanker's skull does not.
The skull and crossbones on the scabbard shown is neither of those.
I can see no attachment method, so I doubt it was actually a bayonet. Though it is possible that it was made from a bayonet blade.
It's also possible some previous owner attached the skull & crossbones disc to play pirate. You might have someone's "play" dagger, made from some original military knife or dagger, or even a bayonet, that just happened to fit the available scabbard.
Anything is possible, and I'm no bayonet expert. The pictured item lacks any of the features I see on late 19th and early 20th century bayonets. It is more similar to a "dress dagger" but is not any recognizable type, to me.
Good Luck getting an ID, sorry I can't tell you what it is, only what it's not.