neuter possessive pronoun; the modern word begins to appear in writing at the end of 16c., from it + genitive/possessive ending 's (q.v.), and "at first commonly written it's, a spelling retained by some to the beginning of the 19c." [OED]. The apostrophe came to be omitted, perhaps because it's already was established as a contraction of it is, or by general habit of omitting apostrophes in personal pronouns (hers, yours, theirs, etc.).
The neuter genitive pronoun in Middle English was his, but the clash between grammatical gender and sexual gender, or else the application of the word to both human and non-human subjects, evidently made users uncomfortable. Restriction of his to the masculine and avoidance of it as a neuter pronoun is evidenced in Middle English, and of it and thereof (as in KJV) were used for the neuter possessive. Also, from c.1300, simple it was used as a neuter possessive pronoun. But in literary use, his as a neuter pronoun continued into the 17c.
Old English hit, neuter nominative and accusative of third person singular pronoun, from Proto-Germanic demonstrative base *khi- (cf. Old Frisian hit, Dutch het, Gothic hita "it"), from PIE *ko- "this" (see he). Used in place of any neuter noun, hence, as gender faded in Middle English, it took on the meaning "thing or animal spoken about before."
The h- was lost due to being in an unemphasized position, as in modern speech the h- in "give it to him," "ask her," "is only heard in the careful speech of the partially educated" [Weekley]. It "the sex act" is from 1610s; meaning "sex appeal (especially in a woman)" first attested 1904 in works of Rudyard Kipling, popularized 1927 as title of a book by Elinor Glyn, and by application of It Girl to silent-film star Clara Bow (1905-1965). In children's games, meaning "the one who must tag the others" is attested from 1842.
: Clara Bow, the original it girl
1. Incompatible time-sharing System
An influential but highly idiosyncratic operating system written for the PDP-6 and PDP-10 at MIT and long used at the MIT AI Lab. Much AI-hacker jargon derives from ITS folklore, and to have been "an ITS hacker" qualifies one instantly as an old-timer of the most venerable sort. ITS pioneered many important innovations, including transparent file sharing between machines and terminal-independent I/O. After about 1982, most actual work was shifted to newer machines, with the remaining ITS boxes run essentially as a hobby and service to the hacker community. The shutdown of the lab's last ITS machine in May 1990 marked the end of an era and sent old-time hackers into mourning nationwide (see high moby). The Royal Institute of Technology in Sweden is maintaining one "live" ITS site at its computer museum (right next to the only TOPS-10 system still on the Internet), so ITS is still alleged to hold the record for OS in longest continuous use (however, WAITS is a credible rival for this palm).
2. A mythical image of operating system perfection worshiped by a bizarre, fervent retro-cult of old-time hackers and ex-users (see troglodyte). ITS worshipers manage somehow to continue believing that an OS maintained by assembly language hand-hacking that supported only monocase 6-character filenames in one directory per account remains superior to today's state of commercial art (their venom against Unix is particularly intense).
See also holy wars, Weenix.