Wiktionary
plumb crazy
a. (context informal English) Thoroughly crazy; quite mad.
Usage examples of "plumb crazy".
As plumb crazy as it was, with Uncle John getting killed almost two days after Tom and with both of 'em obviously dying almost the instant they was shot, it turned out no one thought anything of it.
The crowd had gone plumb crazy, fighting to get at the headache cure.
Well, that's when I realized how plumb crazy he was, because he jumped so high he banged his head on a limb and then ran off into the woods, astumbling every which way and spouting out more gibberish.