Etymology
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fort (n.)

mid-15c., "fortified place, stronghold," from Old French fort "fort, fortress; strong man," noun use of adjective meaning "strong, stout, sturdy; hard, severe, difficult; hard to understand; dreadful, terrible; fortified" (10c.), from Latin fortis "strong, mighty; firm, steadfast; brave, spirited," from Old Latin forctus, which is of unknown etymology. Possibly from PIE root *bhergh- (2) "high, elevated," with derivatives referring to hills and hill-forts, or possibly from *dher- "to hold firmly, support." Figurative use of hold the fort attested from 1590s.

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fortissimo (adj.)
1724, from Italian fortissimo, superlative of forte "loud, strong," from Latin fortis "strong" (see fort).
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forte (adj.)
music instruction, "loud, loudly," from Italian forte, literally "strong," from Latin fortis "strong" (see fort). Opposed to piano.
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fortitude (n.)

late 14c., "moral strength (as a cardinal virtue); courage," from Latin fortitudo "strength, force, firmness, manliness," from fortis "strong, brave" (see fort). From early 15c. as "physical strength."

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forte (n.)

1640s, fort, from French fort "strong point (of a sword blade)," earlier "fort, fortress" (see fort). Meaning "strong point of a person, that in which one excels," is from 1680s. Final -e- added 18c. in imitation of Italian forte "strong."

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aqua fortis (n.)
also aquafortis, old commercial name for "diluted nitric acid," c. 1600, Latin, literally "strong water;" see aqua- + fort. So called for its power of dissolving metals (copper, silver) which are unaffected by other agents.
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Fort Sumter 
military installation in South Carolina, U.S., begun in 1827, named for U.S. Revolutionary War officer and Congressman Thomas Sumter (1734-1832), "The Carolina Gamecock." The family name is attested from 1206, from Old French sommetier "driver of a pack horse" (see sumpter). The U.S. Civil War is held to have begun with the firing of rebel batteries on the government-held fort on April 12, 1861.
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fortify (v.)
early 15c., "increase efficacy" (of medicine); mid-15c., "provide (a town) with walls and defenses," from Old French fortifiier (14c.) "to fortify, strengthen," from Late Latin fortificare "to strengthen, make strong," from Latin fortis "strong" (see fort) + combining form of facere "to make" (from PIE root *dhe- "to set, put"). Sense of "to strengthen mentally or morally" is from late 15c. Meaning "add liquor or alcohol" is from 1880; meaning "add nutrients to food" is from 1939. Related: Fortified; fortifying.
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fortress (n.)

early 14c., from Old French forteresse, forterece "strong place, fortification" (12c.), variant of fortelesse, from Medieval Latin fortalitia, from Latin fortis "strong" (see fort) + -itia, added to adjectives to form nouns of quality or condition. French -ess from Latin -itia also is in duress, largesse, riches, also obsolete rudesse, "lack of cultivation" (early 15c.).

For change of medial -l- to -r- in Old French, compare orme "elm" from Latin ulmus; chartre from cartula; chapitre from capitulum.

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comfort (v.)

late 13c., conforten "to cheer up, console, soothe when in grief or trouble," from Old French conforter "to comfort, to solace; to help, strengthen," from Late Latin confortare "to strengthen much" (used in Vulgate), from assimilated form of Latin com-, here perhaps an intensive prefix (see com-), + fortis "strong" (see fort).

The change of -n- to -m- began in English 14c. In Middle English also "give or add strength to" (c. 1300); "encourage, urge, exhort" (c. 1300). Related: Comforted; comforting.

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