TOD: There are so many words that amuse me that it's hard to choose. Lots of musical terms are funny to me: hypophrygian or hypomixolydian, semiquaver, melismatic...
Colloquialism is also fun to say. I saw a play once (can't remember the name but I'm dure our DR's would know) where, as a running joke, a character kept referring to words as a "local colloqualism". I liked the way that rolled off the tongue, or rather, ensnared by the tongue.
In the local colloquialism department, Greg and I often adopt words that are misprounounced by our neighbors: words like "chimbley" (chimney) and "whelps" (welts). There are so many and we use them so often that we forget that this is not how the words are pronounced. I'm waiting for the day when someone hears us talking this way and thinks we're "ignunt".