Green’s Dictionary of Slang

dis adj.

[orig. naut. use; ult. telegraphist’s jargon dis, disconnected]

eccentric, mentally unstable.

1921		N&Q 12 Ser. IX 425: Gone Dis. [...] Generally employed to indicate vacuity of mind.
1937–84		Partridge DSUE (1984) 313/2: since ca. 1919.