Spirantization is a phonological process by which a plosive sound (or stop) becomes a fricative in the same place of articulation. An example is the shift from P to F in many Germanic languages during the Antiquity. In modern Hiberno-English (English as spoken in Ireland), some words ending in T, such as "right" or "cat", have become spirantized. The tongue moves to the same position in the mouth, but does not fully make contact with the soft palette to close off the flow of air. The resulting consonant is sometimes represented in print as SH, which it superficially resembles.
A familiar example is the pronunciation of "don't you" as "don chu".