In Latin, it simply means "an amount", which can be of anything and of any size. In modern English, especially in physics, it means the smallest amount physically possible, i.e. a physically indivisible amount of something, especially pertaining to energy.
If you look at Max Planck's 1900 paper (originally in German: Zur Theorie des Gesetzes der Energieverteilung im Normalspectrum) where etymonline claims he introduced the term, he didn't just use "quantum," he used "elementary quantum" (Elemantarquantum in German); that is, the most basic (i.e. smallest) quantity of electric charge.
And Max Planck wasn't even the one who introduced the term in German; I can find quite a few occurrences of Elementarquantum in the 1880s and 1890s, and a few before that.
Rather than using "elementary quantum" every time they wanted to refer to one of these things, physicists started abbreviating the term. Already in this 1905 paper of Einstein's (for which he got the Nobel Prize), he just uses the words Energiequant and Lichtquant (energy quantum and light quantum in English). And at some point in the early 1920s, nearly all physicists stopped using the term "elementary quantum" entirely and called these things "quanta". You can see this happen in this Google Ngram.
The idea of a “small quantity” was introduced at the beginning of the 20th c. by German physicist Max Planck:
The word was introduced in physics directly from Latin by Max Planck, 1900, on the notion of "minimum amount of a quantity which can exist;" reinforced by Einstein, 1905. Quantum theory is from 1912; quantum mechanics, 1922.