"long wooden lever for propelling a boat," Middle English or, from Old English ar, from Proto-Germanic *airo (source also of Old Norse ar, Danish aare, Swedish åra), a word of unknown origin. Apparently unrelated to the IE root that is the source of Latin remus "oar," Greek eretēs "rower," eretmos "oar," English row (v.) and rudder. As "oar-like appendage of an animal," 1580s.
A long oar, used occasionally to assist a vessel in a calm, is a sweep, and is operated by two or more men. Small oars are sculls; one rower wielding a pair, sitting midlength of the thwart. ["Knight's American Mechanical Dictionary," 1884]
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit aritrah "oar;" Greek eressein "to row," eretmon "oar," trieres "trireme;" Latin remus "oar;" Lithuanian iriu, irti "to row," irklas "oar;" Old Irish rome "oar," Old English roðor "rudder," rowan "go by water, row."
mid-15c. (late 12c. as a surname), a variation or alteration of Middle English rother, from Old English roðor "paddle, oar," from Proto-Germanic *rothru- (source also of Old Frisian roðer, Middle Low German roder, Middle Dutch roeder, Dutch roer, Old High German ruodar, German Ruder "oar"), from *ro- "steer" (from PIE root *ere- "to row") + suffix *-þra, used to form neutral names of tools.
The original sense is obsolete. The 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 movement turned -d- to -th- in father (n.), etc.
c. 1400, padell "small, long-handled spade used to remove earth adhering to a plow," probably from Medieval Latin padela, a word of uncertain origin, perhaps from Latin patella "small pan, little dish, plate," diminutive of patina (see pan (n.)). Meaning "short oar with a wide blade" (or two, one at each end) is from 1620s. As an instrument used for beating clothes (and slaves, and schoolboys), it is recorded from 1828, American English. As "fin-like forelimb of a sea creature," by 1835. Paddle-ball is attested from 1935.
Old English hilt "hilt, handle of a sword or dagger," from Proto-Germanic *helt (source also of Old Norse hjalt, Old High German helza "hilt," Old Saxon helta "oar handle"), of uncertain origin, possibly from PIE root *kel- "to strike, cut" (see holt). Formerly also used in plural in same sense as singular. Up to the hilts "completely" is from 1670s.