At first glance, I thought marrow might be Latin, because it's medical terminology, but on further inspection, it definitely gave off Germanic vibes. The way the consonants formed around the vowels just sort of had an Old English ring to it... and I was right! There are attestations going back to before Old English, and as such, there was a lot of orthographic variation. Over time, spellings such as merg, mearh, mearg, meari, marwz, mergh, maree, merow, merch, marowe, maree, and eventually marrow emerged (in that order of attestation). All of this is reconstructed as coming from the Proto-Germanic root mazgaz, with the same definition, and that in turn is thought to derive from Proto-Indo-European mosgos, which could also mean "brain" and resulted in some Slavic cognates meaning "mind" (such as mozga in my home language of Serbian!) Since its first use in the late 1300s, the word marrow peaked in the 1990s and has been decreasing in usage since.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.