Is it ensure, insure, or assure?
1660s, "subservient, subordinate," from Latin ancillaris "relating to maidservants," diminutive of ancilla "handmaid," fem. diminutive of anculus "servant," literally "he who bustles about," from root of ambi- "about" (see ambi-) + PIE *kwol-o-, from root *kwel- "move round, turn about, be much about" (see cycle (n.)).
ancillary an·cil·lar·y (ān'sə-lěr'ē)
Relating to or being auxiliary or secondary.