couldn'tで始まる言葉

couldn't

could not の短縮形

can't [couldn't] do better than do

…するのがいちばんよい

Couldn't be better.

（調子・気分は）最高だ

couldn't [((米))could] care less

((略式))（…には/…かどうか）まったく関心がない［気にしない］≪about/wh節，if節≫I couldn't care less who goes.だれが行こうとおれの知ったことか

(I) couldn't.

((略式))（飲食物を勧められて）もう結構です