WASHINGTON — In a decision last week in a patent case, Chief Justice John G. Roberts Jr. puzzled out the meaning of a federal law by consulting the usual legal materials — and five dictionaries.

One of the words he looked up was “of.” He learned that it means pretty much what you think it means.

In May alone, the justices cited dictionaries in eight cases to determine what legislators had meant when they used words like “prevent,” “delay” and “report.” Over the years, justices have looked up both perfectly ordinary words (“now,” “also,” “any,” “if”) and ones you might think they would know better than the next guy (“attorney,” “common law”).

All of this is, lexicographers say, sort of strange.

“I think that it’s probably wrong, in almost all situations, to use a dictionary in the courtroom,” said Jesse Sheidlower, the editor at large of the Oxford English Dictionary. “Dictionary definitions are written with a lot of things in mind, but rigorously circumscribing the exact meanings and connotations of terms is not usually one of them.”

In the last two decades, the use of dictionaries at the Supreme Court has been booming.

A new study in The Marquette Law Review found that the justices had used dictionaries to define 295 words or phrases in 225 opinions in the 10 years starting in October 2000. That is roughly in line with the previous decade but an explosion by historical standards. In the 1960s, for instance, the court relied on dictionaries to define 23 terms in 16 opinions.

Justices Oliver Wendell Holmes Jr., Benjamin N. Cardozo and Louis D. Brandeis managed to make it through distinguished careers on the Supreme Court without citing dictionaries.

Learned Hand, widely considered the greatest judge never to have served on the Supreme Court, cautioned against the mechanical examination of words in isolation.

See: The New York Times