Sources

“[ˈtʌui] 1403, terras de duabus Tollis A4. 1495, Uvir Towiis and Nethir Towiis RMS. 1512, terras de duabus Tollis RMS. As with the other occurrences of this name, the old spellings are Towie or Tolly indifferently. Since the original word is presumably tulach, a knoll, it may be thought that the change from Tolly to Towie occurred in the older Scots; but the change may be partially Gaelic since to turn l into w was a noticeable habit with Braemar Gaelic speakers.” Alexander, 402

Elsewhere however Towie or Tolly come from Gaelic Tollaidh or Tollaigh ‘at hole-place’ (Watson 1926, 442).