n. 25B6; adjective (informal)
a crazy old man : MAD, insane, out of one's mind, deranged, demented, not in one's right mind, crazed, lunatic, non compos mentis, unhinged, mad as a hatter, mad as a March hare; informal mental, off one's head, nutty (as a fruitcake), off one's rocker, not right in the head, round the bend, raving mad, bats, batty, bonkers, cuckoo, loopy, loony, bananas, loco, with a screw loose, touched, gaga, doolally, not all there, out to lunch, away with the fairies; Brit. informal barmy, crackers, barking, potty, round the twist, off one's trolley, not the full shilling; N. Amer. informal nutso, out of one's tree, meshuga, wacko, gonzo; Austral./NZ informal bushed.
a crazy idea : STUPID, foolish, idiotic, silly, absurd, ridiculous, ludicrous, preposterous, farcical, laughable, risible, nonsensical, imbecilic, hare-brained, half-baked, impracticable, unworkable, ill-conceived, senseless; informal cock-eyed; Brit. informal barmy, daft.
he's crazy about her : PASSIONATE ABOUT, very keen on, enamoured of, infatuated with, smitten with, devoted to; very enthusiastic about, fanatical about; informal wild/mad/nuts about, gone on; Brit. informal potty about.
sane, sensible, apathetic.