Why is the ninth month called September?
early 15c., "one who rings" (a bell), agent noun from ring (v.1). In quoits (and by extension, horseshoes) from 1863, from ring (v.2). Especially in be a dead ringer for "resemble closely," 1891, from ringer, a fast horse entered fraudulently in a race in place of a slow one (the verb to ring in this sense is attested from 1812), possibly from British ring in "substitute, exchange," via ring the changes, "substitute counterfeit money for good," a pun on ring the changes in the sense of play the regular series of variations in a peal of bells (1610s). Meaning "expert" is first recorded 1918, Australian slang, from earlier meaning "man who shears the most sheep per day" (1871).
[fr the expression ring someone in, ''announce or herald someone'']