That “gestures come in clusters, like words in a sentence, and that they must be interpreted in the context in which you observe them” is the golden rule of understanding body language, says ‘The Book of Body Language’: a fantastically comprehensive body language resource, hosted by Westside Toastmasters.
In the chapter on hand and thumb gestures, this in particular piqued my interest:
Research into the Hands Clenched position by negotiation experts Nierenberg and Calero showed that it was also a frustration gesture when used during a negotiation, signalling that the person was holding back a negative or anxious attitude. It was a position assumed by a person who felt they were either not convincing the other person or thought they were losing the negotiation. […]
We discovered a correlation between the height at which the hands are held and the degree of the person’s frustration: that is, a person would be more difficult to deal with when the hands are held high, as in a centre position, than they would be in a lower position. […] As with all negative gestures, you need to take action to unlock the person’s fingers, by offering them a drink or asking them to hold something, or their negative attitude will remain in the same way it does with any arm-crossing position.