1. To become eccentric, odd, or mentally unbalanced. Jackson went completely dotty after spending a month by himself in his cabin.
2. To become forgetful or absent-minded, especially due to senility or old age in general. Grandpa's gone completely dotty lately—I don't know if he can take care of himself anymore.
3. To become very excited or enthusiastic (about something). I've never understood that pop star's popularity, but kids just go completely dotty over her music.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.