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.