Chambers's Twentieth Century Dictionary
M. E. dream, drēm, not recorded in A.S., but pointing to an assumed A.S. dréam, cog. with O. High Ger. troum, O. Norse draum, &c. This is distinct from the A.S. dréam, mirth, minstrelsy, being ultimately related to dreug-, draug-, drug-, to deceive, the radical sense therefore 'illusion.'