'To tread upon' definitions:

Definition of 'To tread upon'

From: GCIDE

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. [1913 Webster]
Where'er you tread, the blushing flowers shall rise. --Pope. [1913 Webster]
Fools rush in where angels fear to tread. --Pope. [1913 Webster]
The hard stone Under our feet, on which we tread and go. --Chaucer. [1913 Webster]
2. To walk or go; especially, to walk with a stately or a cautious step. [1913 Webster]
Ye that . . . stately tread, or lowly creep. --Milton. [1913 Webster]
3. To copulate; said of birds, esp. the males. --Shak. [1913 Webster]
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. [1913 Webster]
One woe doth tread upon another's heel. --Shak. [1913 Webster]