"anything that hangs down from a point of attachment and is free to swing;" specifically, in mechanics, "a body so suspended from a fixed point as to move to and fro by the alternate action of gravity and its acquired energy of motion," 1660, from Modern Latin pendulum (1643), noun use of neuter of Latin adjective pendulus "hanging down," from pendere "to hang, cause to hang" (from PIE root *(s)pen- "to draw, stretch, spin"). The Modern Latin word is perhaps a Latinization of Italian pendolo.
"the order being changed," c. 1600, Latin, from vice, ablative of vicis "a change, alternation, alternate order" (from PIE root *weik- (2) "to bend, to wind") + versa, feminine ablative singular of versus, past participle of vertere "to turn, turn about" (from PIE root *wer- (2) "to turn, bend"). "The phrase has the complete force of a proposition, being as much as to say that upon a transposition of antecedents the consequents are also transposed" [Century Dictionary].
As a noun from 1610s, "tool or instrument for drawing angles and adjusting abutting surfaces;" 1670s as "an angle between adjacent sides." The verb, "to reduce to a sloping edge," is first recorded 1670s. Related: Bevelled; bevelling.
The sense evolved to "a composition (usually from Scripture) set to sacred music" (late 14c.), then "song of praise or gladness" (1590s). It came to be used in reference to the English national song (technically, as OED points out, a hymn) and extended to those of other nations. Modern spelling is from late 16c., perhaps an attempt to make the word look more Greek.
"aftermost fore-and-aft sail of a three-masted ship," early 15c., mesan, via French misaine "foresail, foremast," altered (by influence of Italian mezzana "mizzen") from Old French migenne, from Catalan mitjana, from Latin medianus "of the middle" (from PIE root *medhyo- "middle").
The sense of the English and Italian words agree, but the etymology is off because the "middle" mast on a ship is the mainmast. Perhaps it refers to a sail of "middle" size, or the thing described changed. Klein suggests an alternate etymology of the French word, from Arabic via Italian. The mizzen-mast (late 15c.) supports the mizzen-sail.
also flip flop, "plastic thong beach sandal," by 1970, imitative of the sound of walking in them. Flip-flap had been used in various senses, mostly echoic or imitative of a kind of loose flapping movement, since 1520s:
Flip-flaps, a peculiar rollicking dance indulged in by costermongers, better described as the double shuffle; originally a kind of somersault. [Hotten's Slang Dictionary, 1864]
Flip-flop in the general sense of "complete reversal of direction" dates from 1900; it began to be used in electronics in the 1930s in reference to switching circuits that alternate between two states. As a verb by 1897. Flop (n.) in the sense "a turn-round, especially in politics" is from 1880.