Why is the ninth month called September?
Old English swingan "to rush, fling oneself," from Proto-Germanic *swenganan (cf. Old Saxon, Old High German swingan, Old Frisian swinga, German schwingen "to swing, swingle, oscillate") denoting "violent circulatory motion." The meaning "move freely back and forth" is first recorded 1540s. Related: Swung; swinging. Swing shift first recorded 1941, typically 4 p.m. to midnight.
late 14c., "a stroke with a weapon," from swing (v.). Sense of "an apparatus that swings" is first recorded 1680s. Meaning "shift of public opinion" is from 1899. The meaning "variety of big dance-band music with a swinging rhythm" is first recorded 1933, though the sense has been traced back to 1888; its heyday was from mid-30s to mid-40s. Phrase in full swing "in total effect or operation" (1560s) probably is from bell-ringing.