Is it ensure, insure, or assure?
late 14c., from Old French conspirer (14c.), from Latin conspirare "to agree, unite, plot," literally "to breathe together," from com- "together" (see com-) + spirare "to breathe" (see spirit (n.)). Or perhaps the notion is "to blow together" musical instruments, i.e., "To sound in unison." Related: Conspired; conspiring.