Peh = 80, ordinal 17, reduced 8, final form ף = 800. Spelled in full, פה = 80+5 = 85.
Meaning
Peh is 'mouth', one of the seven doubles (p/ph), and the letter of speech; Sefer Yetzirah already classes the letters by the organ that makes them, and the medieval grammarians build their whole phonetic system on it. Its 80 is the Psalm's 'fourscore' span of human life, and its final form 800 is the third of the five hundreds.