great folksy word
it means 'talk/reason too much', 'talk superfluously'
давай, не рассусОливай!- с'mon, don't talk too much (do your job or hurry up)
originates from сус(о)лить, an obsolete synonym of сосать/обсасывать - suck (on)
ground squirrel is called сУслик, probably due to their typical gesture as if they suck on something
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а rude equivalent of рассусоливать is жевать сопли - chew on the snot