late 14c., "profound depth," from Old French golf "a gulf, whirlpool," from Italian golfo "a gulf, a bay," from Late Latin colfos, from Greek kolpos "bay, gulf of the sea," earlier "trough between waves, fold of a loose garment," originally "bosom," the common notion being "curved shape." This is from PIE *kuolp- "arch, curve, vault" (compare Old English hwealf"vault," a-hwielfan "to overwhelm," Old Norse holfinn "vaulted," Old High German welban "to vault").
Latin sinus underwent the same development, being used first for "bosom," later for "gulf" (and in Medieval Latin, "hollow curve or cavity in the body"). The geographic sense "large tract of water extending into the land" (larger than a bay, smaller than a sea, but the distinction is not exact and not always observed) is in English from c. 1400, replacing Old English sæ-earm. Figurative sense of "a wide interval" is from 1550s. The U.S. Gulf States so called from 1836. The Gulf Stream (1775) takes its name from the Gulf of Mexico.