(n.) a feeling of internal anguish or anxiety that shows in someone’s face
Also variously known as the mullygrubs, mulligrumphs, molligrubs and murdiegrups, to be in your merlygrubs is to be so internally vexed or upset that your dispirited feelings show in your face—or, in the words of one nineteenth century dictionary, “to have an internal ailment which evidences itself by contortions of the features.”
If that sounds more like a gastric problem than an emotional one, there’s probably good reason. Etymology, the “grubs” at the heart of the merlygrubs are precisely that: maggot-like larvae that would infest stored foodstuffs, and make all those unfortunate to consume them unwell. Back in eighteenth century English, such vermin were none-too-affectionately nicknamed mullygrub-gurgeons.
That earlier and somewhat specific meaning quickly gave way to a more figurative one, but the notion of an internal upset that distorts someone’s appearance remained in place. Since the late 1500s at least, merlygrubs has been used of any momentary period of upset or melancholy that clearly presents itself in a person’s visage.