Jamieson, Fausset and Brown on Daniel 4:25

25. they shall drive thee--a Chaldee idiom for "thou shalt be driven." Hypochondriacal madness was his malady, which "drove" him under the fancy that he was a beast, to "dwell with the beasts"; Da 4:34 proves this, "mine understanding returned." The regency would leave him to roam in the large beast-abounding parks attached to the palace.

eat grass--that is, vegetables, or herbs in general (Ge 3:18).

they shall wet thee--that is, thou shalt be wet.

till thou know, &c.--(Psa 83:17, 18; Jr 27:5).