arbor (n.)youdaoicibaDictYouDict[arbor 词源字典]
c. 1300, herber, "herb garden," from Old French erbier "field, meadow; kitchen garden," from Latin herba "grass, herb" (see herb). Later "a grassy plot" (early 14c., a sense also in Old French), "a shaded nook" (mid-14c.). Probably not from Latin arbor "tree," though perhaps influenced by its spelling.

The change from er- to ar- before consonants in Middle English also reflects a pronunciation shift: compare farm from ferme, harbor from Old English herebeorg.[arbor etymology, arbor origin, 英语词源]