I've used this word for years and think nothing of it.
Then I started learning characters. "mǎshàng" in characters is composed of two characters: "馬" meaning "horse" and "上" meaning "above, on". Essentially this means, "on horse".
"I will go to the store immediately." = "I will go the store on horse."
"She arrived immediately after I called her." = "She arrived on horse after I called her."
I chuckle over this when I do things "immediately", wondering back to the time when doing something on horseback was extremely fast and about as "immediate" as one could expect.
In our day and age of supersonic airplanes and couple hundred horsepower cars, doing something by horse is probably not the way to do things in a timely manner. I marvel at this relic in Chinese language and enjoy to see how the word has progressed to its current meaning.
The other day, I was joking around about needing something "immediately" in the English language. "I need to get to work on that posthaste." I thought about it and realized that it had the exact same meaning and context as it's equivalent in Chinese.
Back in the day, the mail service was the fastest method out there for communication and delivery of goods. In our day of email and airmail, even skype, chat and twitter, the speed of the mail in relation is now colloquially referred to as "snail mail".
Isn't language a fascinating thing?