This is now, with disambiguation, topping my favorites list. From the OED:
..on conscionable: "Found with its compounds, and CONSCIONED, in first half of 16th c. These, with CONSCIONLESS, appear to be popular formations from conscion, taken as a singular of conscien-ce (see note to the latter)."
on conscien-ce: "In the 15-16th c., the word appears to have been often, by reason of its final s sound, associated with plurals like wits, brains, bowels, as patience is still in dialects. Cf. the illiterate spellings consions, conchons, etc.: hence apparently conscion- in CONSCIONABLE, CONSCIONED, CONSCIONLESS. "
I love it. Definitely used in speech (I use it a few times a year, without thinking about it), and the origin of some great com-fauxing-pounds, but invisible in print!