Words related to set
mid-14c., "a distinctive system of beliefs or observances held by a number of persons; a party or school within a religion," from Old French secte, sete "sect, religious community" (14c.) and directly from Late Latin secta "religious group, sect in philosophy or religion," especially a heretical one. This is a special development of Latin secta "manner, mode; following; school of thought; course, system," literally "a way, a road, a beaten path," from fem. of sectus, variant past participle of sequi "follow," from PIE root *sekw- (1) "to follow." General sense of "those of a certain way of thinking or living" is from late 14c.
The notion in the Late Latin development is "those following (someone's) way." But the history of the word seems to be confused with that of Latin secta, fem. past participle of secare "to cut" (from PIE root *sek- "to cut"). The meaning "separately organized religious body, denomination" is recorded from 1570s in a Protestant context and seems to carry more of a notion of a party "cut off" from a main body.
It also was used in Middle English generally of a class of people or things, a species or race, a distinctive costume, sometimes also of sex (perhaps partly by confusion with that word).
see set (n.1).
The extra t is an arbitrary addition in various technical senses, from a lawn-tennis to a granite set. Each class of persons has doubtless added it to distinguish the special sense that means most to it from all others ; but so many are the special senses that the distinction is now no more distinctive than an Esq. after a man's name, & all would do well to discard it. [Fowler]
With past tense sat, formerly also set, now restricted to dialect, and sate, now archaic; and past participle sat, formerly sitten. In reference to a legislative assembly, from 1510s. Meaning "to baby-sit" is recorded from 1966.
To sit back "be inactive" is from 1943. To sit on one's hands was originally "to withhold applause" (1926); later, "to do nothing" (1959). To sit around "be idle, do nothing" is 1915, American English. To sit out "not take part" is from 1650s. Sitting pretty is from 1916.
Old English had setnes, which was pressed into service in Old and Middle English (setnesse) to translate various ideas in Roman law and Christianity: "foundation, creation, construction; size, extent; statute or law, ordinance; constitution; instruction; sentence; pre-arranged time." But modern use seems to be a fresh coinage of the 17c. The notion in the older word is "what is decreed or determined." Compare German Gesetz "a law, statute," which is related to setzen "to make sit, set, put" and other examples under law (n.)).