Marcel Duchamp was the class clown of the school of modern art. When he drew a mustache on a picture of the ''Mona Lisa,'' he proved that art could be spun from pure adolescent insolence. No other artist accomplished so much by seeming to do so little. The artfulness of his anti-art antics won him a loyal following in the 1960's, when the art scene took a detour from high-mindedness and became a prankster parade. To Duchamp we owe our current Age of Irony and a culture in which nothing is taken more seriously than the gravely hip joke.

Calvin Tomkins's ''Duchamp'' -- the first full-dress biography of the artist -- puts a dark cloak on its bemused and elusive subject. Mr. Tomkins, a longtime writer at The New Yorker and the author of an earlier monograph on Duchamp, befriended the artist in the early 60's. Duchamp was then living in New York, revered as a Dadaist deity whose most absurd creation was surely his own life. It had been four decades since he first announced (not quite dishonestly) that he was giving up art for chess. Callers to his downtown apartment invariably found him in his armchair, a lean, striking Frenchman puffing a cigar and insisting that he was simply too lazy to resume making art. When people asked him how he spent his time, he replied that he was a ''respirateur,'' a breather, as if that were enough to keep even the most wayward genius content.