As Sabrina says, the meaning seems to be that one is out of whack or screwy, i.e. the mental machinery is missing a few cogs and wheels :-)

The Chambers Thesaurus has some wonderful synonyms for "insane", take your pick! Maybe one that keeps the technical undertone.

mad, lunatic, unbalanced, psychotic, disturbed, deranged, maniacal, out of your mind, out of your senses, of unsound mind, unhinged, crazed, unstable, non composmentis, frenzied, wild, berserk, manic, maniac, distracted, distraught, fey, frenetic, frantic, stone-crazy, queer; Scot gyte, red-mad, old frantic-mad, horn-mad, lymphatic, bestraught; (Shakespeare) wood; (Spenser) yond

colloq. crazy, demented, nuts, nutty, nutty as a fruitcake, wacky, mad as a hatter, barmy, bonkers, batty, cracked, crackers, dippy, daffy, dotty, loopy, potty, off your nut, off your head, wrong in the head, out of your head, off the wall, out to lunch, round the bend, round the twist, bats, having bats in the belfry, cuckoo, off the rails, screwy, up the wall, raving, not all there; N American buggy, flaky, fruity; Aust & NZ dingbats

slang. loony, mental, bananas, barking, wacko, doolally, off your rocker, off your chump, off your trolley, out of your tree, needing your head examined, having lost your marbles, having a screw loose, having a tile loose, having several cards short of a full deck, with one sandwich short of a picnic, meshuga, ape, apeshit; N American gonzo, loco, wiggy