/kUd/ modal verb 3rd person singular couldnegative short form couldn't
1 the past tense of `can': Could you hear that all right? | I couldn't get tickets after all, they were sold out. | Marcia said we could smoke, it was okay with her.-see can 12 used to ask if someone is able or allowed to do something: Could I ask you a couple of questions? | What about Sam? Could he come along too?3 used to express something that might be possible or might happen, when it is not certain that it will happen or be possible: Most accidents in the home could be easily prevented. | It could be weeks before the construction is actually finished. | If you're not careful, you could find yourself without enough stock to fill the order.4 used to be polite when you are asking someone to do something: Could you pay this check into the bank for me tomorrow? | Yeah, there are a couple of things you could do for me if you're going into town.5 used to suggest what you think someone should do or might be able to do: We could get the bus instead. | You could always try phoning her at the office. | If you could let us know your decision as soon as possible, it would be a great help.6 used to show that you are annoyed about something: You could have told me you were going to be late! | I'm sure John could be more careful when he's washing up. | How could you say such an insulting thing to her! She's my best friend!7 I couldn't care less used when you are not at all interested in or concerned about something: I said I couldn't care less if I got paid triple time, I'm not coming in on a Sunday. | A lot of the students just couldn't care less about learning anything.8 I couldn't agree more used when you completely agree with someone: I couldn't agree more. There's just far too much sex and violence on TV.9 I could have strangled/hit/killed etc sb used to emphasize that you were very angry with someone: I could have murdered Ryan for telling Jason that!10 couldn't be more wonderful/exciting/boring etc also couldn't be better/prettier/worse etc used to emphasize how good, exciting etc something is: It couldn't have been a more restful vacation. | Things couldn't be worse, everything seems to be going wrong at once.11 I couldn't used to politely say that you do not want any more food or drink: "Would you like another piece of pie?" "Oh, no thanks, I couldn't."
Longman dictionary of contemporary English. 2004.