"piece of rubber behind the wheel of a vehicle to prevent mud from splashing," 1903, from mud (n.) + flap (n.).
Entries linking to mud-flap
mud n.
late 14c., mudde, "moist, soft earth," cognate with and probably from Middle Low German mudde, Middle Dutch modde "thick mud," from Proto-Germanic *mud- from PIE *(s)meu-/*mu- [Buck], found in many words denoting "wet" or "dirty" (source also of Greek mydos "damp, moisture," Old Irish muad "cloud," Polish muł "slime," Sanskrit mutra- "urine," Avestan muthra- "excrement, filth"); related to German Schmutz "dirt," which also is used for "mud" in roads, etc., to avoid dreck, which originally meant "excrement." Welsh mwd is from English. The older word is fen.
Meaning "lowest or worst of anything" is from 1580s. As a word for "coffee," it is hobo slang from 1925; as a word for "opium" from 1922. Mud-puppy "salamander" is by 1855, American English; the mud-dauber wasp was so called by 1856. The children's mud-pie is attested from 1788. Mud-flat "muddy, low-lying ground near a shore" is by 1779. Mud-room "room for removing wet or muddy footwear" is by 1938.
The expression clear as mud (that is, "not clear at all") is by 1796. To throw or hurl mud "make disgraceful accusations" is from 1762. To say (one's) name is mud and mean "(one) is discredited" is recorded from 1823, from mud in obsolete sense of "a stupid twaddling fellow" (1708). Mud in your eye as a toast is recorded from 1912, American English.
flap n.
mid-14c., flappe "a blow, slap, buffet," probably imitative of the sound of striking. Sense of "device for slapping or striking" is from early 15c. Meaning "something that hangs down" is first recorded 1520s, probably from flap (v.). Sense of "motion or noise like a bird's wing" is 1774; meaning "disturbance, noisy tumult" is 1916, British slang.