There are shadowy associations to this word. It comes from a Latin verb that itself derives from umbra, a shadow, which has also given us umbrella, sombre (US somber), and umbrage.
All the English senses have figurative associations with dimness or shade. The principal one today is “report or represent in outline”, to sketch dimly in words, one might say, which is very close to the sense of the Latin. If it’s not a word in your working vocabulary, that’s hardly a surprise, since it has always tended to turn up in academic or formal prose:
Feeble is human speech to deal with such high matters, serving, at the best, but dimly to adumbrate ineffable truths.
The Contemporary Review, January 1883.
It can also mean to indicate something faintly or merely hint at it, to foreshadow or prefigure a future event, or to overshadow or obscure something. Here’s an example of the hinting sense:
Perhaps Lessing’s point, merely adumbrated, is that the long Edwardian afternoon would have entailed a continuation of the great Edwardian philanthropy, otherwise brutally curtailed.
The Spectator, 24 May 2008.