Definition of "truism"

The phrase If something exists, there is porn of it used to be a clever internet meme; now it’s a truism.

2018 November 13, Kate Julian, “Why Are Young People Having So Little Sex?”, in The Atlantic‎

After a little decent sorrow had been put forth for the late "severe affliction," joined with some weariful truisms about resignation to the will of Providence, the rest of the epistle was filled up with reproofs about her ladyship's extraordinary and improper conduct in setting off without his consent!

1837, L[etitia] E[lizabeth] L[andon], “The Remembrance of the Dead”, in Ethel Churchill: Or, The Two Brides. […], volume II, London: Henry Colburn, […], page 321