The source of this word is – most likely – Welsh, where cibi or cibwst means exactly the same thing as Shakespeare’s four “kibes”. That meaning evidently has something to do with feet, as the Fool poses Lear the curious question, “If a man’s brains were in’s heels, were’t not in danger of kibes?” Lear’s “Ay, boy” suggests that he agrees with this. It is not until we look in The Tempest, though, that we can get a clearer picture of this phenomenon, when Antonio offers both podiatric advice and a defence of his guiltless coup against Prospero:
SEBASTIAN. I remember
You did supplant your brother Prospero.
ANTONIO. True.
And look how well my garments sit upon me;
Much feater than before; my brother’s servants
Were then my fellows; now they are my men.
SEBASTIAN. But, for your conscience,–
ANTONIO. Ay, sir; where lies that? If ’twere a kibe,
‘Twould put me to my slipper: but I feel not
This deity in my bosom: twenty consciences
That stand ‘twixt me and Milan, candied be they
And melt ere they molest!
Kibes here, although used metaphorically to illustrate the absent pangs of Antonio’s conscience, obviously suggest something uncomfortable or painful to do with feet, making walking best performed in slippers. The OED confirms this with the meaning of “a chapped or ulcerated chilblain, especially on the heel”, and its entry suggests also that this may be something of a vulgar word, given that it can also be used for damage to the hooves of sheep and horses.
Two occurrances of the word remain in Shakespeare’s opus. Pistol shows absolutely no sympathy for Falstaff’s being “almost out at heels” but rather tells him “Why, then, let kibes ensue”. Similarly, Hamlet picks up on the vulgarity of “kibes” when he remarks to Horatio that “these three years I have taken note of it, the age is grown so picked that the toe of the peasant comes so near the heel of the courtier he galls his kibe”, where to “gall a kibe” means something like to tread on someone’s toes (or rather, ‘ulcerated chilblains’), and thus the larger metaphor describes the Prince’s distaste for either social climbers or – more probably – the debased Danish nobility.
To conclude this foray into Shakespeare’s boots, I’d like to point out that there is a great and longstanding relation between tragedy and feet. It begins with Oedipus, of course, and pops up again with Philoctetes. Perhaps the Bard knew this, and so, by having characters high and low complain about their kibes, further inscribed himself in the Greek classical tradition. After all, what could be a more bathetic hamartia than a blister?