Origin and history of illth

c. 1200, "wickedly; with hostility," from ill (adj.). Meaning "not well, poorly" also is from c. 1200. It generally has not shifted to the realm of physical sickness, as the adjective has done. Ill-fated recorded from 1710; ill-informed from 1824; ill-tempered from c. 1600; ill-starred from c. 1600. Generally contrasted with well, hence the useful, but now obsolete or obscure illcome (1570s), illfare (c. 1300), and illth.

mid-12c., welth, "state or condition of happiness, well-being, joy" (contrasted with care or woe, a sense now obsolete), also "valuable material possessions; prosperity in abundance," from wele "well-being" (see weal (n.1)) on analogy of health (see -th (2)).

By 1590s as "plenty or abundance" of anything. Wealth of nations can be found in Dryden (1666); Adam Smith's book is from 1776. Wealth-tax is by 1963.

suffix forming nouns of action, state, or quality from verbs or adjectives (such as birth, bath, depth, death, growth, strength, truth, math (n.2)), from Old English -ðu, -ð, from Proto-Germanic *-itho (cognates: Old Norse -þ, Old High German -ida, Gothic -iþa), abstract noun suffix, from PIE *-ita (cognates: Sanskrit -tati-; Greek -tet-; Latin -tati-, as in libertatem "liberty" from liber "free"). Sometimes in English reduced to -t, especially after -h- (as in height).

Formerly more widespread (Middle English had stilþe "silence," c. 1200; wrengthe "wrongness, crookedness, distortion," c. 1300), and in recent centuries often tempting to new coinages (17c. swelth "swelling;" Ruskin's illth).