1540s, "grow together, unite by growing into one body," from Latin coalescere "unite, grow together, become one in growth," from assimilated form of com- "together" (see co-) + alescere "be nourished," hence, "increase, grow up," inchoative of alere "to suckle, nourish," from PIE root *al- (2) "to grow, nourish." Related: Coalesced; coalescing; coalescence; coalescent.
