An ambulocetus is a remarkably fat person. This person is beyond whale fat. the word comes from the /fit/ board on 4chan. an ambulocetus had been a sizable land mammal that evolved into whales. it is often called the hiking whale, so fatasses are now known as ambulocetus's on /fit/ Literally "walking whale", it really is a mean-spirited method of explaining an obese person. Basically, it is a stealthy or mocking pseudo-technical method of saying "landwhale".