Symbolism, Imagery, Allegory
Calling someone an eater of left-overs seems to be a pretty harsh burn among the play's Yoruba characters. So what gives? Think about it this way: If a person only gets to eat table scraps, then they are probably pretty poor and, as such, pretty low on the social status spectrum. And being at the bottom socials means a person doesn't get much respect. So eater of leftovers, then, is a clever way of saying nobody likes you, basically.
This zinger first pops up when Amusa (on the orders of Simon Pilkings) goes to the market to try to intervene in preparations for the ritual suicide. Furious that he's interfering, the girls taunt him mercilessly—and when Iyaloja tries to get them to dial it back a little, one girl exclaims: "Then tell him to leave this market. This is the home of our mothers. We don't want the eater of white left-overs at the feast their hands have prepared" (3.60). Burn, right? Also: They're calling him a traitor—he doesn't just eat leftovers, he eats the leftovers of white people, who oppress the Yoruba. So double burn.
Unfortunately, the insult is eventually directed at Elesin himself when he fails to pull off the ritual suicide. His own son, Olunde, is the one who takes this swipe, saying, "I have no father, eater of left-overs" (4.209), when he finds his father alive and captured by the local British authorities. It's clear that this isn't a tactical rejection of Elesin—Olunde isn't just trying to create distance for hiswell-being or anything like this—it is rooted in a total loss of respect.
When Iyaloja visits Elesin in prison, she uses this turn of phrase yet again to drive home the awfulness of what he's done: "You have betrayed us. We fed you sweetmeats such as we hoped awaited you on the other side. But you said No, I must eat the world's left-overs […] No, I am content to lick the dregs from each calabash when the drinkers are done" (5.78). In other words, Elesin gave up the massive rewards awaiting him on other side to stick around the earthly world—which, in the end, is kind of the ultimate leftover, in the view of his people.