of flower blossoms, "fully open," 1640s, from full (adj.) + blown "that has blossomed," from Old English geblowenne, past participle of blow (v.2) "to bloom." Figuratively "complete, fully developed" from 1650s. Full-blown also was used 17c.-18c. in reference to cheeks, sails, bladders, "fully distended" (by or as if by wind), in this case from blow (v.1), and the figurative sense might also be from or influenced by these.
updated on October 10, 2017