Ap is the seventeenth letter of the Mandaic alphabet and writes p. It joins on both sides, so it carries a connected run onward.
With the affrication mark beneath it, ap stands for f, a sound the inherited alphabet had no separate letter for and which Arabic and Persian loan words bring in constantly. The affrication mark is one of three diacritics in the script, the others marking vowel quality and gemination, and all three may be left off in ordinary writing and supplied by the reader.
Seventeenth place is where pe stands in the Aramaic order, and it is one of the twenty-two places that answer letter for letter to the older Aramaic sequence. Every addition that takes the alphabet past twenty-two comes after the twenty-second sign, so the inherited block is unbroken.
Its name follows the alphabet's usual pattern, and the mouth that stands behind Syriac pe and Hebrew peh has no counterpart in it.
Syriac handles the same pair of sounds differently. There, pe is one of six letters carrying a hard and a soft reading governed by the grammar of the word, and the soft reading is f. In Mandaic the f is written by marking the letter, which is a borrowing device rather than a grammatical alternation. Same two sounds, two quite different pieces of machinery.