bachelor: [13] The ultimate origins of bachelor are obscure, but by the time it first turned up, in Old French bacheler (from a hypothetical Vulgar Latin *baccalāris), it meant ‘squire’ or ‘young knight in the service of an older knight’. This was the sense it had when borrowed into English, and it is preserved, in fossilized form, in knight bachelor. Subsequent semantic development was via ‘university graduate’ to, in the late 14th century, ‘unmarried man’.
A resemblance to Old Irish bachlach ‘shepherd, peasant’ (a derivative of Old Irish bachall ‘staff’, from Latin baculum, source of English bacillus and related to English bacteria) has led some to speculate that the two may be connected. English baccalaureate [17] comes via French baccalauréat or medieval Latin baccalaureātus from medieval Latin baccalaureus ‘bachelor’, which was an alteration of an earlier baccalārius, perhaps owing to an association with the ‘laurels’ awarded for academic success (Latin bacca lauri meant literally ‘laurel berry’).
bachelor (n.)
c. 1300, "young man;" also "youthful knight, novice in arms," from Old French bacheler, bachelor, bachelier (11c.) "knight bachelor," a young squire in training for knighthood, also "young man; unmarried man," and as a university title, of uncertain origin, perhaps from Medieval Latin baccalarius "vassal farmer, adult serf without a landholding," one who helps or tends a baccalaria "field or land in the lord's demesne" (according to old French sources, perhaps from an alteration of vacca "a cow" and originally "grazing land" [Kitchin]). Or from Latin baculum "a stick," because the squire would practice with a staff, not a sword. "Perhaps several independent words have become confused in form" [Century Dictionary]. Meaning in English expanded early 14c. to "young unmarried man," late 14c. to "one who has taken the lowest degree in a university." Bachelor party as a pre-wedding ritual is from 1882.