Webster’s Revised Unabridged Dictionary · 1913
Subsesqui-
Sub·ses′qui- (sŭb·sĕs′kwĭ·). [Pref. sub- + sesqui-.] (Chem.) A prefix (also used adjectively) denoting the combination of constituents (especially electro-negative and electro-positive bodies) in the proportion of two to three; as, a subsesqui acetate, i. e., a salt having two equivalents of acetic acid to three of the base.