1550s, "being in a sound state;" also "conducive to health," from health + -y (2). Earlier in the same sense was healthsome (1530s). Related: Healthily; healthiness.
It is wrong to say that certain articles of food are healthy or unhealthy. Wholesome and unwholesome are the right words. A pig may be healthy or unhealthy while alive; but after he is killed and becomes pork, he can enjoy no health, and suffer no sickness. [Eliza Leslie, "Miss Leslie's Behaviour Book," Philadelphia, 1839]
Healthsome is from 1530s in the sense "bestowing health."