1. To overcome someone's doubt or disbelief about something: "The financial advisor assured Mildred that her investment was safe."
2. To cause someone to feel confident: "Manfred assured his wife that he loved her and he would always be with her as long as they lived."
3. Etymology: from Old French aseurer, "to make sure"; from Vulgar Latin assecurar, from Latin ad-, "to" + securus, "sure, safe, secure".

The Latin word securus also gives us "secure".