rod (n.) Look up rod at
Old English rodd "a rod, pole," which is probably cognate with Old Norse rudda "club," from Proto-Germanic *rudd- "stick, club," from PIE *reudh- "to clear land."

As a long, tapering elastic pole for fishing, from mid-15c. Figurative sense of "offshoot" (mid-15c.) led to Biblical meaning "scion, tribe." As an instrument of punishment, attested from mid-12c.; also used figuratively for "any sort of correction or punishment," but the basic notion is of beating someone with a stick.

As a unit of measure (5½ yards or 16½ feet, also called perch or pole) first attested mid-15c., from the stick used to measure it off. As a measure of area, "a square perch," from late 15c., the usual measure in brickwork. Meaning "light-sensitive cell in a retina" is from 1866, so-called for its shape. Slang meaning "penis" is recorded from 1902; that of "gun, revolver" is from 1903.