Is it ensure, insure, or assure?
1630s, "to plunge or sink in," from Latin mergere "to dip, dip in, immerse, plunge," probably rhotacized from *mezgo, from PIE *mezg- "to dip, plunge" (cf. Sanskrit majjati "dives under," Lithuanian mazgoju "to wash"). Legal sense of "absorb an estate, contract, etc. into another" is from 1726. Related: Merged; merging. As a noun, from 1805.