An indisputable fact or basic truth, especially one that is discomforting to acknowledge.
According to the dictionary, this is from the early 1700s.
Using Google books, it is possible to find uses of the phrase from that time period;
the following is from 'Characteristics of men, manners, opinions, times' by Anthony Ashley Cooper Shaftesbury, which was originally published in 1711:
"If he has indiscreetly spoken some home-truth, or discovered some secret which strikes at the temporal interests of certain spiritual societies, he is quickly doom'd to calumny and defamation."