Is it ensure, insure, or assure?
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'']