我一时还不知道
off the top of my head 是个固定短语“没仔细考虑”（不要孤立去理解off)
.I can't tell you the answer off the top of my head.
我不经考虑无法告诉你答案.
I answered his question off the top of my head.
我没有认真考虑就回答了他的问题.