O.E.
ranc "proud, overbearing, showy," from P.Gmc.
*rankaz (cf. Dan.
rank "right, upright," Ger.
rank "slender," O.N.
rakkr "straight, erect"), perhaps from PIE
*reg- "to stretch, straighten" (see
right). In ref. to plant growth, "vigorous, luxuriant," it is recorded from c.1250. Sense evolved in M.E. to "large and coarse" (c.1300), then, via notion of "excessive and unpleasant," to "having a strong bad smell" (1529). Much used 16c. as a pejorative intensive (cf.
rank folly). This is possibly the source of the verb meaning "to reveal another's guilt" (1929, underworld slang), and that of "to harass, abuse," 1934, U.S. black dialect, though this also may be from the role of the activity in establishing social hierarchy (from
rank (n.)).
[rahngk] 
oʊ