also back-seat, 1832, originally of coaches, from back (adj.) + seat (n.). Used figuratively for "less or least prominent position" by 1868. Back-seat driver "passenger who gives the driver unwanted advice" is attested by 1923.
You know him. The one who sits on the back seat and tells the driver what to do. He issues a lot of instructions, gives advice, offers no end of criticism and doesn't do a bit of work. ["The Back Seat Driver," Wisconsin Congregational Church Life, May 1923]
c. 1400, "keep from view or use, render inaccessible" early 15c., "to lock up, confine," from shut (v.) + up (adv.). The meaning "cause to stop talking" is from 1814 (Jane Austen). The intransitive meaning "cease from speaking" is from 1840, also as a command to be silent, sometimes colloquialized in print as shuddup (1940). Put up or shut up "defend yourself or be silent" is U.S. slang, by 1868.
"add strength," 1941, from college slang, from beef (n.) in slang sense of "muscle-power" (1851).