"No thanks. I appreciate it [your offer of coffee]. But I'm good". That would be the equivalent translation to colloquial English.I appreciate the comments which have already appeared here, but if I may, I'll add one more example of the use of عشت, which I found in my ancient "Egyptian Colloquial Arabic", by a chap named Mitchell. A customer in a shop has been offered coffee by the owner, but the customer says لا كتر خيرك, قهوتك مشروبة, عشت. I wish I could understand how to translate all this into good, colloquial English.