To chew or chomp noisily, or to move around slowly and heavily.
Likely of imitative origin, related to 'mumble' and 'fumble,' with the 'ch' sound mimicking the sound of chewing. English has many such sound-words that emerged organically in the language.
Words like 'chumble' are onomatopoeia—language that sounds like what it means—which is why babies learning English instinctively understand them before anyone teaches them.
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