Origin of discreet
Discrete, “apart or detached from others; separate; distinct,” was originally a spelling doublet of discreet, sharing meanings and both deriving from the same Latin source. The spelling discrete is closer in form to Latin discrētus and is probably an attempt to differentiate discreet from discrete.
The Middle English Dictionary has two senses at the headword discrēt(e): the sense “wise, morally discerning, prudent, circumspect” has 29 citations; the sense “separate, distinct” has four (in angelology, astronomy, and mathematics).
It wasn’t until the late 16th century that discrete became restricted to its present meanings, leaving the spelling discreet to predominate in its own current uses.
Examples from the Web for discreetness
You have not taken the discreetness I have shown into consideration, although you were interested in remaining silent.Ten Years Later|Alexandre Dumas, Pere
Would they be boilin' us the night wid the discreetness of 'em?The Belted Seas|Arthur Colton
You may depend upon me to be discreetness itself, at least until after Dick has been safely shipped.The Snare|Rafael Sabatini
You were discreetness itself, and the man you called Barney was a capital actor, and a rare good fellow too.Out of a Labyrinth|Lawrence L. Lynch
Milray seemed to feel the note of discreetness in her laugh, and he asked, smiling, "How old did you tell me you were?"Ragged Lady, Complete|William Dean Howells