He spaced out for a while and didn't hear what the teacher was saying.
他一时走神，没有听到老师所说的话。
space out是比较口语的说法，意思是一时走神，一时失神。space out 也可指使（人）昏昏沉沉或头脑不清，如：
That medication totally spaced me out.
那药物让我完全神智不清。
