As explained in the following extract, the meaning of disgusting was not a big semantic jump from the original meaning and usage of gross. This connotation appears to have become popular as a slang term among teenagers in the ‘60s/‘70s, but its earliest usage appears to date from 1958:
- Meaning "disgusting" is first recorded 1958 in U.S. student slang, from earlier use as an intensifier of unpleasant things ( gross stupidity , etc.) (Etymonline)
The word gross has been in English for hundreds of years. We got it from French, where it means "big" or "fat." It took on a variety of senses in English related to size, including "coarse" (gross grains as opposed to fine), "strikingly obvious" (grosse as a mountaine), and "whole" (gross as opposed to net value). It also picked up negative senses like "vulgar," "crude" (Grose folke of rude affection Dronkerdes. Lubbers, knaues), or "ignorant" (a grosse unlettered people).
From there it’s not a big jump to the current sense of disgusting. There’s always been something repulsive, or at least unsavory, in the word gross.
Gross did not undergo a big change in meaning, but it did undergo a big change in context. In the late 20th century, young people started to use it a lot—like, a lot a lot. So much so that old people noticed it, and didn’t like it. As one critic said in a 1971 issue of The Saturday Review, “Gross has always meant something coarse and vulgar. But as used by the teens, it runs the gamut of awfulness from homework to something the cat contributed to ecology.” Gross became slang.
Note also the usage of gross-out:
something that is disgustingly offensive.
(First recorded in 1970–75; noun use of verb phrase gross out)
(Random House Unabridged Dictionary)
Edit by WS2:
Supplementary to this answer, and also that of @Laurel, reproduced below is sense 15 of the adjective gross per the current online edition of the OED. Clearly the "student slang" to which Etymoline refers is closely related here.
The current online edition of the OED includes the following - to which the "student slang"
15. Extremely coarse in behaviour or morals; brutally lacking in refinement or decency.
a. of persons.
?1533 G. Du Wes Introductorie for to lerne Frenche sig. Sii Grose folke of rude affection Dronkerdes..Lubbers,
1667 Milton Paradise Lost i. 491 Belial..then whom a Spirit more
lewd Fell not from Heaven, or more gross to love Vice for it self.
1693 Dryden tr. Juvenal Satires vi. 122 Agamemnon's Wife Was a
gross Butcher, with a bloody Knife.
1772 E. Burke Corr. (1844) I. 402 The Turks..grow more gross in
the very native soil of civility and refinement.
1881 Evans in Sp. Com. 1 Cor. Introd. 239 Society of high culture,
but in morals lax, even gross. absolute.
b. of habits, language, pleasures, etc.
1598 Shakespeare Love's Labour's Lost i. i. 29 The grosser manner
of these worldes delyghts: He throwes vppon the grosse worlds baser
1725 D. Defoe New Voy. round World i. 169 The Motive of their
Adoration being that of meer Terror, they have certainly gross Ideas.
1791 J. Boswell Life Johnson anno 1749 I. 103 [Paraphrasing
Johnson:] Some of them [sc. Juvenal's Satires]..were too gross for
1877 ‘Rita’ Vivienne i. i. 15 Of life in its grosser, harsher
phases Albert knew scarce anything.