early 14c., "country-man, peasant farmer, rustic," from Old French bovier "herdsman," from Latin bovis, genitive of bos "cow, ox." This was reinforced by or merged with native Old English gebur "dweller, farmer, peasant" (unrelated but similar in sound and sense), and 16c. by its Dutch cognate boer, from Middle Dutch gheboer "fellow dweller," from Proto-Germanic *buram "dweller," especially "farmer" (compare German Bauer), from PIE root *bheue- "to be, exist, grow."
"A word of involved history in and out of English, though the ultimate etymology is clear enough" [OED]. In English it often was applied to agricultural laborers in or from other lands, as opposed to the native yeoman; negative transferred sense "one who is rude in manners" attested by 1560s (in boorish), from notion of clownish rustics. Related: Boorishness.