Thank you very much! As for a non-native English speaker, I found myself really have trouble understanding these casual words, although I got full marks on reading part. I mistook his words and thought this "sit on" means "choose" or "stick to".
I have been waiting for 6 weeks after being placed in the waitlist at Dartmouth. I applied for 8 PhD programs and got 1 waitlist and 7 reject. This is my only hope now. I asked the POI about the information, he said "I am still waiting to hear back from both candidates. I have a feeling that they are going to sit on their offer till April 15 and I cannot do anything about it.". What does it means? It seems that the hope is slim.