1642: PIE root of thumb Jun 13, 2019

The word 'thumb' and 'tumor' have a distant etymological relation, but many other languages no longer share this root too. 'Thumb' is from Old English, but 'tumor' comes from Latin meaning 'to swell'. Indeed the Latin, Greek, Sanskrit words etc. for 'thumb' are each very different looking, but each of those languages, and many other Indo-European languages have that share this root and mean totally different things, like the Greek 'τύμβος' meaning 'burial mound, or the Sanskrit तुम्र (túmra), 'thick'. Each of these have different definitions, but they all relate to the same sorts of things: unevenness or swelling.
See more on 'thumb' here.