Origins
The word 'adult' entered English in the 1530s from Latin 'adultus,' the past participle of the verb 'adolēscere,' meaning 'to grow up' or 'to come to maturity.' The Latin verb is composed of the prefix 'ad-' (toward) and 'alēscere,' an inchoative (beginning) form of 'alere' (to nourish, to feed). The underlying idea is that growing up is a process of being nourished toward completion. The ultimate source is the Proto-Indo-European root *h₂el-, meaning 'to grow' or 'to nourish.'
The relationship between 'adult' and 'adolescent' is one of the neatest etymological pairs in English. Both words come from the same Latin verb 'adolēscere,' but they represent different grammatical stages. 'Adolescent' comes from 'adolēscēns,' the present participle — someone who is in the process of growing up. 'Adult' comes from 'adultus,' the past participle — someone who has completed the process. The two words are literally 'growing' and 'grown,' a before-and-after snapshot from a single verb.
The PIE root *h₂el- was highly productive. Through Latin 'alere' (to nourish), it gave English 'aliment' (food, nourishment), 'alimentary' (relating to nutrition), 'alimony' (literally 'nourishment money'), and 'alma mater' (literally 'nourishing mother,' a term for one's university). Through Latin 'alumnus' (a foster child, one who is nourished), it gave the modern academic term. Through Latin 'altus' (high, literally 'grown tall'), it produced 'altitude,' 'alto,' 'exalt,' and 'altar' (a high place). Through Old English, the same root may have produced 'old' (from Proto-Germanic '*aldaz,' literally 'grown'), though this derivation is debated.
Latin Roots
English initially borrowed 'adult' as an adjective meaning 'grown up, mature,' and the noun use (an adult person) followed within a few decades. The word remained fairly formal and technical until the eighteenth century, when it began to displace older English words like 'grown-up' in educated speech. In the twentieth century, 'adult' acquired a euphemistic sense relating to sexually explicit material ('adult entertainment,' 'adult bookstore'), first attested in the 1950s.
The stress pattern of 'adult' varies between dialects. In British English, the stress traditionally falls on the first syllable (/ˈæd.ʌlt/), while in American English it more often falls on the second (/əˈdʌlt/). Both pronunciations are considered standard. The adjective 'adulting,' meaning 'behaving like a responsible adult,' emerged as internet slang around 2008 and entered mainstream dictionaries by the 2010s, reflecting a cultural shift in how younger generations relate to the concept of adulthood.
Despite the superficial similarity, 'adultery' has no etymological connection to 'adult.' It comes from a different Latin word entirely: 'adulterāre,' meaning 'to corrupt, to falsify, to debase,' which itself is of uncertain origin but may relate to 'alter' (other). The resemblance between 'adult' and 'adultery' is a coincidence of spelling that has occasionally led to folk-etymological confusion.