barnburner A radical, zealot, or extremist; historically, a member of the radical faction of the Democratic party in New York State (1840-50) so eager for political reform that he would through excess of zeal destroy what he wished to preserve. The term, which dates from 1841, comes from the older phrase, burn a barn to kill the rats, in use since 1807.

eager beaver A ball of fire, an especially industrious or zealous person; an excessively aggressive or ambitious person, a go-getter. This American expression, dating from the mid-1900s, is a reference to the beaver’s reputation for being particularly hardworking and diligent. Earlier similar phrases include work like a beaver ‘work very hard or industriously,’ which dates from the early 18th century; and industrious or busy as a beaver ‘very busy,’ in use since the early 19th century.

gung ho Wholeheartedly enthusiastic; eager, zealous, patriotic, loyal. Gung ho is a corruption of the Chinese kung ho ‘work together’ (kung ‘work’ + ho ‘together’). The unit of United States Marines that served under General Evans F. Carlson in World War II adopted the expression as its slogan. The phrase appeared in its original form, kung-hou, as early as 1942.

hellbent Recklessly dogged or stubbornly determined; resolute, persistent; going at breakneck speed. The term, of American origin, dates from at least 1835. It has spawned the expanded forms hellbent for leather, hellbent for election, and hellbent for breakfast. Hell or hellbent for leather, thought to be originally British but popular on both sides of the Atlantic, has only the second sense of hellbent, i.e., going at tremendous speed. The reference is to riding on horseback, leather referring to the leather of the saddle. The expression is found in Rudyard Kipling’s The Story of the Gadsbys, published in 1888. Hellbent for election is said to have originated in the Maine gubernatorial race of 1840. Hellbent for breakfast, dating from at least 1931, is another expanded form of hellbent; it is used in the second sense only—going at great speed.

whirling dervish A person who vociferously expounds his opinions and beliefs; a zealot. A dervish is an Islamic priest or monk who, during religious ceremonies and prayers, frequently enters a type of ecstatic rapture marked by wild dancing, violent movements, and loud singing or chanting. Thus, these holy men came to be known as whirling dervishes or howling dervishes.

The expression is applied in non-Islamic contexts by extension.