The kīla(nail) or phurba is a three-sided peg, stake, knife, or nail like ritual implement traditionally associated with Indo-Tibetan Buddhism, The kīla is associated with the meditational deity Vajrakīla or Vajrakīlaya
Most of what is known of the Indian kīla lore has come by way of Tibetan culture. Scholars such as F. A. Bischoff, Charles Hartman and Martin Boord have shown that the Tibetan literature widely asserts that the Sanskrit for their term phurba is kīlaya (with or without the long i). However, as Boord describes it, “all dictionaries and Sanskrit works agree the word to be kīla (or kīlaka). I suppose this [discrepancy] to result from an indiscriminate use by Tibetans of the dative singular kīlaya. This form would have been familiar to them in the simple salutation namo vajrakīlaya (homage to Vajrakīla) from which it could easily be assumed by those unfamiliar with the technicalities of Sanskrit that the name of the deity is Vajrakīlaya instead of Vajrakīla. It should also be noted that the term (vajra)kīlaya is frequently found in Sanskrit texts (as well as in virtually every kīlamantra) legitimately used as the denominative verb ‘to spike,’ ‘transfix,’ ‘nail down,’ etc.
Mayer (1996) contests Boord’s assertion, pointing out that eminent Sanskritists such as Sakya Pandita employed Vajrakīlaya Further, he argues:
it is possible, on the other hand, that the name Vajrakīlaya as favoured by the Tibetans could in fact have been the form that was actually used in the original Indic sources, and that there is no need to hypothesize a correct form Vajrakīla”. “Vajrakīlaya” could have come from the second person singular active, causative imperative, of the verb Kīl. Indigenous grammar (Pāṇini Dhātupāṭha I.557) gives to Kīl the meaning of bandha, i.e. “to bind”, while Monier-Williams (285) gives the meanings “to bind, fasten, stake, pin”. Hence the form kīlaya could mean “you cause to bind/transfix!”, or “bind/transfix!”. This, taken from mantras urging “bind/transfix”, or “may you cause to bind/transfix”, might have come to be treated as a noun; and the noun might then have become deified; hence Kīlaya might have started out as a deified imperative, in some ways comparable to the famous example of the deified vocative in the name Hevajra, and a not unheard of phenomenon in Sanskrit tantric literature. This suggestion is supported by Alexis Sanderson, a specialist in Sanskrit tantric manuscripts whom I consulted on this problem