late 14c., diffusioun, "a copious outpouring," from Old French diffusion and directly from Latin diffusionem (nominative diffusio) "a pouring forth," noun of action from past-participle stem of diffundere "scatter, pour out," from dis- "apart, in every direction" (see dis-) + fundere "to pour" (from nasalized form of PIE root *gheu- "to pour"). Meaning "act of diffusing, state of being diffuse" is from 1590s; figurative sense of "a spreading abroad, dispersion" (of knowledge, etc.) is by 1750.
