"petty quarrel," 1804, American English, of unknown origin; perhaps somehow imitative (compare spat "smack, slap," attested from 1823).
"short gaiter covering the ankle" (usually only in plural, spats), 1779, shortening of spatterdash "long gaiter to keep trousers or stockings from being spattered with mud" (1680s), from spatter and dash (v.).
"spawn of a shellfish," especially "spawn of an oyster," also "a young oyster," 1660s, of unknown origin, perhaps from the past tense of spit (v.1).
Bullets were spatting down on us
mollusks or oysters spat
Bullets were spatting the leaves