To arrive at the heart of the matter. Some think this late nineteenth-century term comes from Cockney rhyming slang for hard facts. Another possible and perhaps more likely source is the American general store, where a countertop was marked with brass tacks at one-yard intervals for measuring cloth, and “getting to brass tacks” meant measuring precisely. Still another theory is that in upholstered furniture, brass tacks were used to secure the undermost cloth, and to reupholster properly one had to strip the furniture to that layer. A mid-twentieth-century American synonym is to get down to the nitty-gritty, alluding to the detailed (nitty) and perhaps unpleasant (gritty) facts of the case. It was borrowed from black English, where it signified the anus and alluded to picking body lice (nits) from that body part. This association had been largely forgotten by the time the term was popularized by the 1964 hit song “The Nitty Gritty” by Shirley Ellis.

To take or be on the correct course of action or reasoning. The first expression refers to the tack of a sailing ship— that is, its course when it is tacking (steering in zigzag fashion when sailing to windward). The word “tack” was being transferred to a course of action by 1675. The second term, which dates from about 1880, alludes to the direction of a path. Both have antonyms—on the wrong tack or track—denoting a mistaken course of action or reasoning.

Singularly keen or cutting; also, mentally acute. This simile has largely supplanted the earlier sharp as a razor, needle, vinegar, and thorn, the last dating from the fifteenth century and appearing in John Ray’s 1670 proverb collection. The current cliché dates from the late nineteenth or early twentieth century and appeared in a 1912 issue of Dialect Notes: “They won’t fool him; he’s sharp as tacks.”