means A mound or bank of snow piled up by the wind, especially against an obstacle like a wall, fence, or doorway.

from A plain compound of "snow" and "drift" — and "drift" is the key half, an old Germanic word tied to "drive," meaning literally a thing that has been driven along. So a snowdrift is snow that the wind has driven, herded into heaps, the way "snowdrift" quietly admits it never chose to be there. The same drifting sense survives in driftwood and in cattle being driven across a field.