They started off late.这句话什么意思啊?怎么用了off

started off起身,出发
他们出发的时候已经很迟了
那就是你字典的问题了,看看网上字典的：
start off
vt.出发(动身,开始)
例句与用法:
If you try to start off in top gear you'll stall.
要是刚一发动就用最高挡,那就会熄火.
I start off as a kitchen helper.
我开始是干厨房帮手的活.
Don't light a cigarette when you meet the boss; you'll start off on the wrong foot.
见老板时不要点烟,要不然,你和他一开始就会处不好.
They will soon start off.
他们很快就要动身了.
We can make it if we start off now.
现在我们出发还赶得上.