To parboil is to boil food, usually vegetables, briefly and lightly. To parboil is to partially boil, like someone really sleepy is telling you to “part boil” the parsnips before you roast them.
To boil something, you cook it in hot, steaming water, often for a long time. Parboiling takes boiling down a notch. When you parboil, you boil just a little while, which is enough for some vegetables but for other food that’s just the first step in the cooking process. Sometimes you parboil, also called blanch, food before you freeze it. The Latin root actually meant the opposite, but parboil sounds too much like “part boil,” so there you have it.