late 14c.; see capital (adj.). So called because it is at the "head" of a sentence or word.

also smart alek, smart alec, "would-be clever person," by 1863 ["Weekly Butte Record," Oroville, Calif., May 16, 1863], of unknown origin. Barnhart suggests perhaps in reference to Aleck Hoag, notorious pimp, thief, and confidence man in New York City in early 1840s. But the dates don't overlap and the earliest use seems to concentrate in the U.S. West (Idaho and Nevada). See smart (adj.). Related: Smart-alecky (1905).

The phrase "Smart Aleck" is now of very common occurrence .... [Idaho Statesman, Aug. 9, 1870]

Cambodian capital, literally "mountain of plenty," from Cambodian phnom "mountain, hill" + penh "full."

1680s, Latin, "by the head, by heads," from per (see per) + capita "head" (see capital).

the surname (also Munroe, etc.) is said to be ultimately from the River Roe in Derry, Ireland. James Monroe (1758-1831), the fifth U.S. president, was in office from 1817 to 1825. The Monroe Doctrine (so called from 1848) is a reference to the principles of policy contained in his message to Congress on Dec. 2, 1823. Monrovia, the capital of Liberia, also was named for him at its founding in 1822 by the American Colonization Society.

In terms of national psychology, the Monroe Doctrine marked the moment when Americans no longer faced eastward across the Atlantic and turned to face westward across the continent. [Daniel Walker Howe, "What Hath God Wrought"]