What does Fafnir mean?
Old Norse Fafnir, the dwarf who turned into a dragon guarding a cursed hoard in the Volsung legend. Flag: this is the NAME OF a mythological dragon, not a common word meaning 'dragon'; its own etymology is not securely established, so it qualifies by being a famous dragon-bearer rather than by literal translation.
What does Jormungandr mean?
Old Norse Jormungandr, the Midgard/World Serpent that encircles the earth. From jormun- 'huge, vast' + gandr, an ambiguous word covering 'serpent/staff/magical being', commonly read 'giant serpent'. Flag: a mythological serpent's NAME, and the second element is genuinely ambiguous (also 'giant staff/binding'), so the 'serpent' reading is the usual but not the only one.