E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
If you dont like it, you may lump it. Whether you like to do it or not, no matter; it must be done. Here lump it means to gulp it down, or swallow unwillingly, to put up with it unwillingly but of necessity. Thus we say of medicine, lump it down, i.e. gulp it down. (Danish, gulpen, to swallow.)