Early Noldorin
-garn
suffix. *-made
-carn
suffix. *-made
carbh
noun. deed
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
A suffixal form ᴱN. -carn or -garn of the passive participle agarn for the verb ᴱN. cara- “to make, do” in the Early Noldorin Dictionary of the 1920s, hence meaning something like “-✱made”.
Neo-Sindarin: I think this suffix can be salvaged as a reduction of the later passive participle carnen of S. car- “do, make”, which can be used to form adjectives such as adangarn “man-made”. It does conflict with S. carn “deed”, but I prefer S. carth for this noun form.