Now, I believe that this word comes from South Slavic languages and that it spread northward from the Balkans. Oral and written tradition from Russia, Ukraine, Belarus and Poland, claims that Balkans (Illyricum) was the old Slavic homeland. You can read more about this in my post "St Paul among Slavs".
Illyrians were famous seafarers, pirates, and ship builders. Liburna, a type of small galley used for raiding and patrols, by the Illyrians, later became de-facto standard for light Roman military vessels.
Did Illyrians who migrated northward bring with them the ship building skills? And were Slavic and Norse ships based on these Illyrian vessels.
Liburna ship was named after Liburni, Illyrian people who designed it. But what did they themselves call it? Ladia, Ladja?
Possibly. It is in Illyria that we find, what could be, the oldest written example of the word Ladja (ship). Illyrian coin with a ship and inscription "Ladia" = Ship???