Origin and history of bacillus

"unicellular microorganisms which lack an organized nucleus," and sometimes cause disease, 1847, plural of Modern Latin bacterium, from Greek bakterion "small staff," diminutive of baktron "stick, rod, staff, cudgel." So called because the first ones observed were rod-shaped. Introduced as a scientific word 1838 by German naturalist Christian Gottfried Ehrenberg. A classical plural sometimes also erroneously used as a singular.

The Greek word is from a PIE *bak- "staff used for support, peg" (compare Latin baculum "rod, walking stick;" Irish bacc, Welsh bach "hook, crooked staff;" Middle Dutch pegel "peg, pin, bolt"). De Vaan writes, "Since *b was very rare in PIE, and Celtic shows an unexplained geminate, we are probably dealing with a loanword from an unidentified source."

1620s, "university degree of a bachelor," from Modern Latin baccalaureatus, from baccalaureus "student with the first degree," an alteration of Medieval Latin baccalarius "one who has attained the lowest degree in a university, an advanced student lecturing under his master's supervision but not yet having personal licence."

The Medieval Latin word is of uncertain origin; it likely has been altered by folk etymology or word-play, as if from bacca lauri "laurel berry" (laurels being awarded for academic success). Perhaps it is ultimately from Latin baculum "staff" (see bacillus), which the young student might carry. Or it might be a re-Latinization of bachelor in its academic sense.

In modern U.S. usage, baccalaureate usually is a shortening of baccalaureate sermon (1864), a religious farewell address to a graduating class at an American college. This is from the word's adjectival sense of "pertaining to the university degree of bachelor."