from dissension, and therefore more etymologically spelt dissensious;
but perhaps originally after Old French dissencieux, -tieux, from dissencion, -tion, obs. spellings of dissension;
there are no other English words in -ensious, while -entious is frequent, and naturally associates this word with dissent, dissentient, etc.
1560 - N. Macchiavelli's Arte of Warre, Peter Whitehorne;
see below from E-NED
From: The Poems of William Shakespeare
Published by Munroe, Francis, & Parker, 1808
Venus and Adonis