C15 (in the sense: to vibrate, quiver1): from quaven to tremble, of Germanic origin; compare Low German quabbeln to tremble
"to vibrate, tremble," early 15c., probably frequentative of cwavien "to tremble, shake" (early 13c.), probably related to Low Ger. quabbeln "tremble," possibly of imitative origin.
late Middle English (as a verb in the general sense 'tremble'): from dialect quave 'quake, tremble', probably from an Old English word related to quake.
1400–50; late Middle English quaveren (v.), blend of quake and waver1
[C15 (in the sense: to vibrate, quiver 1 ): from quaven to tremble, of Germanic origin; compare Low German quabbeln to tremble]
Middle English, frequentative of quaven to tremble
First Known Use: 15th century
From Middle English quaveren, frequentative form of quaven, cwavien (“to tremble”), equivalent to quave + -er. Cognate with Low German quabbeln (“to quiver”), German quabbeln, quappeln (“to quiver”).
Quake: All the sources used above agree that quake stems from O.E. cwacian / cweccan.
Setting aside the uncertainty between Old English cwavien and Low German quabbeln being the source, it appears to be true that quaver and quake are related, with, according to ODO, the latter possibly being the source of the former. However, there's no mention of waver anywhere besides dictionary.reference.com.
Google, on the other hand, points to a couple of sources that lend credence to the portmanteau idea.
Julie Tetel Andresen, author and associate professor of English (in 1999) at Duke University states:
The word-formation process du jour in American English is blending, that is, combining two existing words to make a new word. A couple of blends formed in the Middle English period (1150 to 1500) have survived into Modern English, e.g., scrawl (sprout + crawl) and quaver (quake + waver), as well as a couple from Early Modern English (1500 to 1800), dumbfound (dumb + confound) and apathetic (apathy + pathetic).
From the book, English words from Latin and Greek elements by Donald M. Ayers:
Other blends are smog (smoke + fog), radiclib (radical + liberal), slurb (slum +
suburb) and its extension slurbia, Comsymp (Communist sympathizer),
harmolodic (harmonic + melodic), comsat (communications + satellite), simulcast (simultaneous + broadcast), quaver (quake + waver), jargonaut ...
From the book, A Biography of the English Language:
However, among the numerous probable blends from ME are scroll from escrow + roll; scrawl from sprout + crawl; and quaver from quake + waver.
To conclude, while the (online) dictionaries do not mention the possibility of quaver being a blended word, there appear to be a few credible sources that do. There are, however, no authoritative sources that support this claim.
I'll be interested to know what the OED says about this.