馬上

Etymology
From, "on horseback", by extension, "at once; immediately", on account of the horse being the fastest mode of transportation in ancient times.

Adverb

 * 1) at once; immediately; without delay; in a minute; soon
 * 2)  on horseback
 * 1)  on horseback
 * 1)  on horseback

Noun

 * 1) horseback, the back of a horse
 * 2) horseback riding