Is there a single English word to describe when someone has appropriated property that doesn't belong to them unintentionally?
For example, say I borrow a pen from someone and absentmindedly put it in my pocket after I finish writing. I discover the pen much later. Did I steal it, or is there a term with less harsh connotations?
You could probably call it an honest mistake (I know, a bit illogical, but still a common phrase).
Following the precise definition, if you borrowed a pen and then failed to return it as agreed, it would still be borrowed—that is took and used with the intention of returning it.
You could say you walked off with it? The OED defines this as:
to walk off with: to carry away as a prize or plunder; to steal.
but in present usage, it’s a bit broader: it can cover either deliberate or accidental stealing, depending on context. Quickly googling "walked off with" for examples, the second hit is:
$11000 Stolen Scarf Was an Accident. New York socialite says she walked off with it by accident…
— NBC Connecticut
It is not only a question of harshness, but of meaning.
Even though you can find dictionary entries that will say (CALD)
steal
to take something without the permission or knowledge of the owner and keep it
where it seems that it does not matter how you took it (intentionally or not), in actual use "to steal" implies intention as per following definition (MW)
steal
to take the property of another wrongfully and especially as a habitual or regular practice
To say wrongfully implies "bad" intention and does not convey the meaning that you have in mind without further explanation.
I don't know of the word which does; all related words I looked at have this implication except the most basic and widest term: "to take".
I can't think of a single word that would accurately describe the act of accidentally keeping, or taking home, an item that has been willingly lent to you by its owner (or their proxy)... In my opinion, you don't really steal the object until you make the concious decision to never return it to its owner.
However, I suppose that if you happened to honestly forget to return someone's property, you could say that you inadvertently pinched (or nicked) their stuff.
Unfortunately most terms I can think of are equally harsh.
Even misappropriate (to steal something that you have been trusted to take care of and use it for your own good) implies misuse of someone else's property. However it does at least imply that you were originally given the pen by the original owner rather than just stealing it.
The word you are looking for is, "CONVERSION". Basically, the difference is, your intent wasn't to deprive the owner of his/her property. In an extreme case, taking a neighbors car to go joyriding with no intent to damage or cause any loss to its owner.
