Lewis & Short

P, p, the fifteenth letter of the Latin alphabet, the character for which is derived from the ancient form of the Greek II (P or P), as is shown by inscriptions and coins, which exhibit the P in these forms.The P-sound, like the K- and T-sounds, was not aspirated in the ancient language; whence the spelling TRIVMPE for triumphe, in the Song of the Arval Brothers.As an initial, P combines, in pure Latin words, only with the consonants l and r; the combinations pn, ps, and pt belong to words borrowed from the Greek, with the sole exception of the pron. suffix pte.
Hence it often disappears before t; as TOLOMEA, Inscr. Fabr. 9, 438.
It has also been dropped before l in the words lanx, Gr. πλαξ; latus, Gr. πλατύς; later, Gr. πλίνθος, linter, Gr. πλυντήρ, and others (Corss. Ausspr. 1, 114).
As a medial, its combination with s and t was so acceptable to the Latins that ps and pt are often put for bs and bt; so, OPSIDESQVE and OPTENVI in the Epitaphs of the Scipios; and so, too, in later inscrr.: APSENS, APSENTI, SVPSIGNARE, etc., and in MSS.
A final p occurs only in the apocopated volup.For the very frequent interchange of p and b, see under B.
P is put for v in opilio for ovilio, from ovis.
An instance of its commutation with palatals appears in lupus and λύκος, and perhaps also spolium and σκῦλον, spuma and O.H.G. scum, Germ. Schaum, as, on the other hand, equus and ἵππος, palumba and columba, jecur and ἧπαρ; cf., also, the letter Q.
Its commutation with a lingual is shown in pavo and ταώς, and perh. also in hospes and hostis.
P is assimilated to a following f in officina for opificina, and is altogether elided by syncope in Oscus for Opscus.
It is euphonically inserted between ms and mt: sumpsi, sumptum, hiemps for hiems; cf.: exemplum, templum, and late Lat. dampnum.
It is suppressed in amnis for ap-nis from apa = aqua.As an abbreviation, P denotes most frequently the prænomen Publius, but also stands for parte, pater, pedes, pia, pondo, populus, posuerunt, publicus, etc. P. C. stands for patres conscripti, patronus civitatis or coloniae, ponendum curavit, potestate censoriā, etc. P. M. pontifex maximus, patronus municipii, posuit merito. P. P. pater patriae, praepositus, primi pilus, pro parte. P. R. populus Romanus. P. S. pecunia sua.