1726, from in- (1) "not, opposite of" + describable. Related: Indescribably; indescribability (1797). In same sense, Old English had unasecgendlic. Indescribables for "trousers" (1819) was colloquial in England for a generation or so.
We cannot omit here to state, that, some years since, we recollect a rumour in the gallery [of the House of Commons], that Madame de Staël was sitting, en habit d'homme, in a surtout and military indescribables, listening to the debate, under the protection of Sir J. Macintosh. ["Privileges of Women," in Retrospective Review, London, 1824]
waterproof outer coat or cloak, 1836, named for Charles Macintosh (1766-1843), inventor of a waterproofing process (patent #4804, June 17, 1823). The Mcintosh type of apple was named for John McIntosh of Upper Canada, who began selling them in 1835. The surname is from Gaelic Mac an toisich "Son of the chieftain." As a name of a type of computer it is attested from 1982.
In an interview with The Wall Street Journal, [President and CEO A.C.] Markkula said "more than three new products" are scheduled to be announced within the next year. Among them will be a high-end, personal business computer code-named "Lisa" and a limited, less expensive business computer called "Mackintosh." [Computerworld, Oct. 18, 1982]