brainstorm A sudden and powerful thought; a good idea. The concept of forcefulness contained in the storm element seems to be losing ground to that of disorder and chaos, so that brainstorm is now most often used ironically to mean a whimsical or ill-considered notion, a stupid idea.
brown study Absorption in thought; a pensive mood; absent-mindedness. This phrase dates from the early 16th century; the brown of the expression apparently stemmed from brown ‘gloomy.’ Citations indicate that the phrase varies in meaning: it may be used for serious thought; for apparent pensiveness masking actual absent-mindedness; or for simple idle daydreaming. John Crowe Ransom uses the phrase poignantly in “Bells for John Whiteside’s Daughter”:
There was such speed in her little body,
Ana such lightness in her footfall,
It is no wonder that her brown study
Astonishes us all.
a horseback opinion A guess, an offhand impression, a hasty opinion or judgment delivered without “stopping to think,” as though from horseback. Use of this U.S. colloquialism dates from the late 19th century.
I am not here as a judicial authority or oracle. I can only give horseback opinion. (Congressional Record. April 23, 1879)
on the carpet Under consideration or up for discussion. This expression, in use since 1726, comes from
the earlier on or upon the tapis (since 1690), a partial translation of the French sur le tapis On the tablecloth.’ The tablecloth in question is the one covering the council table around which the members meet to discuss items of business.
put on one’s thinking cap To think about or consider; to ponder; to reflect or concentrate. Although “thinking caps”have been mentioned in children’s literature and various legends for hundreds of years, the most likely allusion is to the official headgear which a British magistrate would wear when considering the disposition of a case and when passing sentence. In its figurative use, put on one’s thinking cap clearly implies that the matter at hand merits serious thought.
It is satisfactory to know that the Post Office Department has its thinking cap on. (Daily Chronicle. January, 1903)
sleep on it To contemplate and reflect upon an important proposal, plan, or other matter without making a hasty decision; to consider something overnight before making up one’s mind. This expression, in popular use for centuries, implies that some decisions, particularly portentous ones, merit at least one night of conscious and, while sleeping, subconscious thought.
His Grace … said that he would sleep and dream upon the matter, and give me an answer [in] the morning. (State Papers of Henry VIII. 1519)