Anonymous means with the name kept unknown. An anonymous letter, an anonymous donor, remain anonymous.
It covers both hiding a name deliberately and simply not knowing it: an anonymous sixteenth-century poem. Anonymity is the noun. Common phrases: an anonymous tip-off, on condition of anonymity, an anonymous source. Note Alcoholics Anonymous, where the anonymity is a founding principle; and that anonymous is not quite the same as confidential — confidential means the name is known and protected.
1adjectiveanonymousWith the name kept unknown.
“The donation was anonymous and everybody guessed who.”
anonymous is a state of namelessness.
Greek anonymos is an-, without, plus onyma, a name — the same onyma inside synonym, pseudonym and antonym. English kept the whole Greek word intact rather than translating it.
Latin
anonymus
English, today
anonymous
Built from
Borrowed from Latin anonymus. Borrowed into English around 1600 from Latin anonymus, from Ancient Greek ᾰ̓νώνῠμος (“without name”), from ἀ- (“un-”) with ὄνῠμᾰ, Aeolic and Doric dialectal form of ὄνομᾰ (“name”). English equivalent anonym + -ous, its full etymology being an- + -onym + -ous.
Etymology from Wiktionary, CC BY-SA