From in- +‎ coherency or incoherent +‎ -cy.

incoherency (usually uncountable, plural incoherencies)

1. (uncountable) The quality of being incoherent; lack of coherence.
   • 1686 (indicated as 168⁵⁄₆), R B, “The Conclusion”, in A Free Enquiry into the Vulgarly Receiv’d Notion of Nature; , London: H. Clark, for John Taylor , →OCLC, page 409:

     Haste and Sickness made me rather venture on your good Nature, for the Pardon of a venial Fault, than put myself to the trouble of altering the Order of these Papers, and substituting new Transitions and Connections, in the room of those, with which I formerly made up the Chasms and Incoherency of the Tract, you now receive.

   • 1785, Sophia Lee, The Recess‎, volume III, London: T. Cadell, part VI, page 260:

     Pardon, madam, the haste and incoherency of scrawls penned at so trying a moment.

   • 1886 January 5, Robert Louis Stevenson, Strange Case of Dr Jekyll and Mr Hyde, London: Longmans, Green, and Co., →OCLC:

     "It can make no change. You do not understand my position," returned the doctor, with a certain incoherency of manner.

2. (countable) A thing which is incoherent.
   • 1667, J Evelyn, “To the Reader”, in Publick Employment and an Active Life, with Its Appanages, such as Fame, Command, Riches, Conversation, &c. Preferred to Solitude: . By J. E. Esq; S.R.S.">…], London: J. M. for H Herringman , →OCLC:

     But that vvhich vvould beſt of all juſtifie me, and the ſeeming incoherencies of ſome parts of my Diſcourse, vvould be the noble Authors Piece it ſelf, becauſe of the Antitheſis, and the forms of his Applications.

   • 1757, David Hume, “The Natural History of Religion,” section 11, in Four Dissertations, London: A. Millar, page 70:

     For besides the unavoidable incoherencies, which must be reconciled and adjusted; one may safely affirm, that all popular theology, especially the scholastic, has a kind of appetite for absurdity and contradiction.

   • 1887, William Dean Howells, chapter 1, in April Hopes‎, New York: Harper, page 3:

     he took into his large moist palm the dry little hand of his friend, while they both broke out into the incoherencies of people meeting after a long time.

• incoherence