The Knight of Cups as feelings means - "True Love". Whenever I get him in my readings, I know my beloved is about to be very loving and attentive to me (or I to him!), and sweep me (gently) off my feet
. Unlike what Caridwen wrote, I think this knight - like all knights - acts. He acts on his feelings. Only, he's not Mr Speedy Gonzales - he's no knight of Swords or Wands. He will come to you in his own sweet time, LOL. But full of love. You are his Beloved. He follows his heart. He's not about to rationalise his feelings or weigh them against a football match with the boys, like the Knight of Wands might, or against his career, like the Knight of Pentacles.
The Knight of Cups might also be the feeling that you have to go on a journey because your muse has told you to. The feeling that inspires haunting music and poetry. The feeling you must leave that boring job and start following your dreams.
If this is someone else's feelings you are asking about - don't worry if he doesn't call - he's thinking of you, and when he calls, you'll melt
(but you had better open a good savings plan...because if that's all the guy is, you'll be in trouble!)
Nobody is wholly a Knight of Cups (thank God!), but every man or woman in love has a bit of him in them...