1791, in chemistry, "liquefying in air," from Latin deliquescentem (nominative deliquescens), present participle of deliquescere "to melt away," from de- (see de-) + liquescere "to melt," from liquere "to be liquid" (see liquid (adj.)). Transferred or figurative sense of "apt to dissolve or melt away" is by 1837. Related: Deliquescence.
1781 as a French word in English; naturalized from c. 1800, from French cooking term fondue "a cheese-pudding," literally "melted" (15c.), noun use of fem. of fondu, past-participle adjective from fondre "to melt," from Latin fundere (past participle fusus) "to melt, cast, pour out" (from nasalized form of PIE root *gheu- "to pour").