The Collaborative International Dictionary of English v.0.48:

Reddle \Red"dle\ (r?d"d'l), n. [From Red; cf. G. r["o]thel.
   Cf. Ruddle.] (Min.)
   Red chalk. See under Chalk.
   [1913 Webster] Red dog

WordNet (r) 3.0 (2006):

reddle
    n 1: a red iron ore used in dyeing and marking [syn: ruddle,
         reddle, raddle]