From Proto-Germanic*þūmô, from Proto-Indo-European*tūmon-(“thick finger”), from *tūm-, *tū- ‘swell’. Cognate with Old Frisian thūma (West Frisian tomme), Old Saxon thūmo (Low German Dumen or Duum), Dutch duim, Old High German dūmo (German Daumen), Danish tomme, Swedish tumme; and also with Latin tumēre ‘to swell’, through the Indo-European root.