The Glutton's Glossary by John Ayto says:
The use of the word humbug for a stripy peppermint-flavoured boiled sweet seems to date from the nineteenth century: the Oxford English Dictionary notes it as being 'remembered in common use in Gloucestershire' in the 1820s, while Elizabeth Gaskell in Sylvia's Lover's (1863) explained: 'He had provided himself with a paper of humbugs for the child — "humbugs" being the North-country term for certain lumps of toffy, well-flavoured with peppermint.'
...
It originally meant 'practical joke, hoax' rather than the present, more earnest 'hypocritical sham', and its application to an article of food may be of similar inspiration to trifle.
An 1876 Notes and Queries (the Victorian precursor to Stack Exchange) suggests an etymology:
The kind of sweetmeat called humbug can still
be bought at Taunton. It is a thin, oval-shaped
piece of toffee, with an almond in the middle, and
is, I suspect, so called because, after sucking for a
short time at the toffee, you suddenly find yourself
come to an almond. H. F. BOYD.
Webster's claims that canoodle comes from the German knudeln:
Ger knudeln, to cuddle < or akin to LowG knuddel, a knot, clump, dim. of dial. knude; akin to OHG knodo, OE cnotta, knot
Wiktionary claims “origin unknown”, but it offers two possible origins:
Origin Unknown; compare Swedish knulla (“to fornicate”), German knuddeln (“to cuddle”)
Its earliest use is from a British source in 1859, claiming that the word is American, per the Etymonline link provided by OP.
Most other dictionaries claim “unknown origin” as well, many of them agreeing with Etymonline regarding when it was first used, and where.
Considering the large number of resources checked (all 21 links provided by Onelook), we could assume that the origin is, in fact, unknown, as only 2 out of 21 provided alternatives.
However, the possibility of the word having German origin is relatively high—we know that many Germans lived in the United States, since six million Germans immigrated to the United States between 1820 and WWII. On the other hand, this is mere speculation. It is likely safest to say that the origin is, in fact, unknown.
Best Answer
I found the word tantrum itself in print back to 1675 in Charles Cotton's burlesque, The Scoffer Scoft. In this passage Bacchus recounts to Apollo how he was sodomized by Priapus (Greek god of fertility), who "tilt[ed] his Tantrum at [Bacchus'] Nock":
I searched "Tantrum at my Nock" and found it referenced in Farmer's and Henley's Slang and Its Analogues Past and Present, 1904, which has penis as a second definition of tantrum:
(Slang and Its Analogues also confirms nock as slang for the posterior.)
I can't find any other examples of this euphemistic use of tantrum, but the presence of Bacchus in this passage leads to other clues. From @F'x's "interesting read" link I gleaned this set of definitions of tantrum from Charles Mackay's The Gaelic Etymology of the Languages of Western Europe, 1877:
The blogger of F'x's link dismisses the tarantula connection in favor of the Gaelic lead, but when I searched "tarantula dance" I found this in a 1783 collection of essays:
and this from Freaks and Follies of Fabledom, 1852:
So, if there is this connection between Bacchus and this ecstatic spider-bite dance, it seems plausible that Cotton's use of Tantrum above could be referencing this as well and thus point to an arachnological etymology. On the other hand, pet and hot & heavy work quite well for the euphemism, too.
A note on temper tantrum: I found this phrase back to a 1920 (check) publication referring to a psychiatric case at John Hopkins University from 1918 in Mental Hygiene vol. IV:
Interestingly, I found the words comma-separated as the fifth definition of flink in The English Dialect Dictionary from two decades earlier in 1900:
I think this adjacent use of the words in a such an authoritative work could have easily resulted in the coining of the phrase.