mid-15c. alteration of Middle English rother, from Old English roðor "paddle, oar," from Proto-Germanic *rothru- (cognates: Old Frisian roðer, Middle Low German roder, Middle Dutch roeder, Dutch roer, Old High German ruodar, German Ruder "oar"), from *ro- "steer" (see row, v.) + suffix -þra, used to form neutral names of tools.Meaning "broad, flat piece of wood attached to the stern of a boat and guided by a tiller for use in steering" is from c.1300. For shift of -th- to -d- compare burden, n.1, murder, n.; simultaneous but opposite to the movement that turned -d- to -th- in father, n., etc.