antiquarian is one who collects or studies antiquities, and as a modifier describes what belongs to that pursuit — most familiarly the trade in old books, coins, maps, and objects.
The noun appears around 1600, meaning one versed in the knowledge of ancient things, from Latin antiquarius and behind it antiquus, "ancient, aged, venerable." The near-identical antiquary is slightly older, recorded in the 1580s, and the adjective follows much later, in 1771. Over time both nouns narrowed from the scholar toward the dealer, which is why an antiquarian bookseller today is understood to trade in rare and old volumes rather than to write about them.
An antiquarian is not quite a historian and not quite an archaeologist. The distinction is one of method rather than subject: the antiquarian gathers, catalogues, and describes particulars — an inscription, a coin hoard, a parish record, a curiosity cabinet — without necessarily assembling them into an argument about what happened or why. That is also the source of the word's faint condescension, sharpened by Nietzsche's use of antiquarian history in 1874 for a reverence toward the past that preserves without renewing anything. The charge is only half fair. The particulars antiquarians accumulated, and the records of ownership and origin they kept with them, became the material base that professional history and archaeology later worked from, and the discipline of establishing 📝provenance is largely their inheritance.
"A thorough-paced antiquarian not only remembers what all other people have thought proper to forget, but he also forgets what all other people think proper to remember." ~ Charles Caleb Colton, Lacon, 1825