From the Internet > An idiom Put somebody off their stride> especially British English. Knock/throw/keep somebody off stride > American English. = to make someone unable to do something effectively, by not allowing them to give all their attention to it / to interfere with one's normal and natural progress or rate of progress. Your startling comments put Larry off his stride for a moment/ He was put off his stride by an interruption from the audience/ His testimony threw the defense off stride. The unexpected has interfered with his natural tendency to plan and keep things as planned .

It's with reference to walking as a metaphor. When we walk we have a stride, if we get thrown off our stride it means that we at the very least slow down, stumble or can even fall down.

Thank you so much for your kind explanation ^^ (감사합니다.)

'to take things in one's stride' to ​deal with a ​problem or ​difficulty ​calmly and not to ​allow it to ​influence what you are doing: When you ​become a ​politician, you ​soon ​learn to take ​criticism in ​your stride. 'to be thrown off one's stride' has the opposite meaning