From Germanic 'Heimiric' meaning home ruler; feminine form of Harry/Henry.
Harriett is the feminized form of Harry, which itself derives from the medieval English pronunciation of Henry — rooted in the Old High German Heimirich, a compound of 'heim' (home) and 'ric' (power, ruler), meaning approximately 'ruler of the home' or 'estate ruler.' The feminization of Henry into Henrietta and then its diminutive Harriet (with various spellings including Harriett) became widespread in 18th-century England, particularly after it became fashionable to feminize royal names. The double-t spelling, as in Harriett, signals a distinctly careful, slightly formal orthographic choice — an extra flourish of distinction within an already distinguished name.
The name's most towering historical bearer is unquestionably Harriet Tubman (c. 1822–1913), the escaped enslaved woman who became the most celebrated conductor of the Underground Railroad and later a Union spy, suffragist, and humanitarian. Tubman's Harriet was not the name she was born with — she was named Araminta at birth and adopted Harriet in honor of her mother — which makes her choice of the name an act of self-authorship that adds another dimension to its meaning.
Harriet Beecher Stowe, author of Uncle Tom's Cabin (1852), is another 19th-century bearer who shaped American history. The fictional Harriet Smith in Jane Austen's Emma (1815) gave the name early literary presence. Harriet experienced a mid-century dip but has rebounded strongly in the 21st century, driven partly by nostalgia for Victorian names and partly by ongoing cultural celebration of Harriet Tubman. The variant spelling Harriett, with its doubled consonant, reads as especially considered and classic, suggesting a family who has thought carefully about the history they're invoking.