Learn English – Where does the word “jism” come from

Paul Beale, ed., Partridge's Concise Dictionary of Slang and Unconventional English (1989) offers this entry for the verb snog:

snog, v. 'To make love with repletion of kissing and cuddling; hence, snogging session, making love' (L.A. 1977); very common throughout WW2 [and still, 1983, not ob.: P.B.] I surmise a dial. alteration of snug, cosy, notably as in that snuggling-up which so often preludes a warmer conjunction. —2. Hence, to flirt, or to court, esp in be or come or go snogging: beatniks', adopted, ca. 1959 , ex gen s. (Anderson) The term, esp as be or go snogging, seems to have orig in the RAF, late 1930s (Partridge 1945).

However, Eric Partridge, A Dictionary of Slang and Unconventional English, Fifth Edition (1961) suggests a slightly different etymology:

snogging, be or go. To be or go courting a girl; to be or go love-making: RAF: since ca. 1937. Partridge 1945, 'Snog is perhaps a blend of snug and cod (to flatter or kid a person).'

That same source notes that "snogged up"—supposedly an RAF term dating to circa 1939—means "Smartened up, 'all dressed up.'"

I would say it has onomatopeic roots.

Firstly, this isn't the only word for masturbation to have echoic roots: fap is echoic of the noise of masturbation.

Looking at the entry in the OED, some of the uses spell the word whank, and in fact one example mentions that it might be echoic:

1951 E. Partridge Dict. Slang (ed. 4) 1220/1 Whank, (male) self-abuse: low: from ca. 1870. Perhaps echoic.

Looking through Google books, it shows that whank is used as echoic of bird calls:

For example

"Whank, whank, whank!" the little bird calls.

Also in the wank form:

"Wank, wank, wank", it trumpets, in a musical, nasal tone

Given that the OED's earliest citation (1948, British RAF) refers to a bed, I would think that wank is echoic of the sound of bed boards or mattress springs.