请帮我理解一下“was up at”在以下句子的意思

题目

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

答案

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”