It looks like a Flobert indoor target pistol, made in France...but I can't be sure. I can't read the stamped marking on its triggerguard.
Certainly it is well beaten up. The stock has been ill used, and the ornate buttcap is missing.
If it isn't French, it may be Belgian.
Are there any other visible markings? Are there any markings on the barrel, covered by the stock?