The Japanese even have a word for this – “Aimai na iikata”, which literally means “vague way of talking”. “Minna no Nihongo”, the ultimate “go to” book for beginners introduces this in one of the initial lessons. Mira-san invites Kimura-san for a concert with “Isshyoni ikaga desu ka?(Would you like to go to concert with me?). Kimura-san, who is not interested, doesn’t simply say “No, I have other plans”. She instead says “Ashita wa chotto..”. “Chotto” literally means “a little”. Here, it’s used to politely and not to forget gently decline an invitation. She is probably trying to say “A little tough for me tomorrow” If Mira-san presses further with a “How about day after then?” he is “shitsukoi” or “obstinate”. If Mira-san has good “Kuuki wo yomu”skills (“reading into a situation”skills), he will get the message and close the topic with “sou desu ka. zannen desu ne” (Really..that’s too bad).
The advantage with “chotto”is that you don’t have to quickly make up lies. You don’t have to give any reason why. You simply drag the word”Chotto..” :-).