The headword, attested only here, comes from a compound verb ignored by LSJ, e)kpelmato/w; it is either the present participle, feminine nominative plural, of that verb, or its aorist active infinitive. If it is a participle, the feminine noun presupposed could be e)mba/des or the like.
For the etymology see LSJ s.v. pe/lma, "sole of the foot."
See also pi 995 for the perfect middle/passive participle pepeltw/mena (also in other lexica, and always glossed pepinarwme/na, "having been dirtied"), as if from a verb pelta/w or pelto/w. Again LSJ does not recognise such a verb, but does include pelmato/omai, where the entry reads: 'become worn in the sole, prob. in Phot., Suid s.v. pepeltwme/na, (due to misreading of LXX Jo. 9.5)'. In that passage, Joshua 9.5 LXX, the inhabitants of Gibeon try to fool Joshua by wearing sanda/lia ... palaia\ kai\ katapepelmatwme/na, "footwear old and much-cobbled".