also flyblown, "tainted with the eggs of flies in large numbers," of meat, 1570s, earlier in figurative use, "putrid, impure" (1520s). Compare flyblow (n.) "egg of a fly" (1550s); see fly (n.1), also blow-fly.
The sense of blow (v.1) here is said to be "deposit eggs, infect with eggs" in reference to insects, "apparently connected with old notions of natural history" [OED, 1989]. But to blow on or upon meaning "breathe (infectious breath, poison) upon; infect with disease, taint" is by c. 1300, and compare Middle English elf-blown "tainted." Blown also was used in other compounds for "stale from exposure to air," which flows into the senses of "tainted, unsavory; exposed to flies."