The Collaborative International Dictionary of English v.0.48:

Tread \Tread\, v. i. [imp. Trod; p. p. Trodden, Trod; p.
   pr. & vb. n. Treading.] [OE. treden, AS. tredan; akin to
   OFries. treda, OS. tredan, D. & LG. treden, G. treten, OHG.
   tretan, Icel. tro?a, Sw. tr[*a]da, tr[aum]da, Dan. tr[ae]de,
   Goth. trudan, and perhaps ultimately to F. tramp; cf. Gr. ? a
   running, Skr. dram to run. Cf. Trade, Tramp, Trot.]
   1. To set the foot; to step.
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            Where'er you tread, the blushing flowers shall rise.
                                                  --Pope.
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            Fools rush in where angels fear to tread. --Pope.
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            The hard stone
            Under our feet, on which we tread and go. --Chaucer.
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   2. To walk or go; especially, to walk with a stately or a
      cautious step.
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            Ye that . . . stately tread, or lowly creep.
                                                  --Milton.
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   3. To copulate; said of birds, esp. the males. --Shak.
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   To tread on or To tread upon.
      (a) To trample; to set the foot on in contempt. "Thou
          shalt tread upon their high places." --Deut. xxxiii.
          29.
      (b) to follow closely. "Year treads on year."
          --Wordsworth.

   To tread upon the heels of, to follow close upon. "Dreadful
      consequences that tread upon the heels of those allowances
      to sin." --Milton.
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            One woe doth tread upon another's heel. --Shak.
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The Collaborative International Dictionary of English v.0.48:

Trod \Trod\,
   imp. & p. p. of Tread.
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