请帮我理解一下“was up at”在以下句子的意思
问题描述：
请帮我理解一下“was up at”在以下句子的意思
We were all laughing up our sleeves at the teacher when he was up at the blackboard explaining the math problem
答
was up at在这句话里面没有实际意义,属于口语化的词,后半句的意思是,”当他在黑板上解释数学难题时“
所以was up at可以作为“...在...的时候”