1. A small distinguishing mark on the letters "i" or "j". It was originally a larger mark, first appearing in Latin manuscripts, but was shortened when Roman-style typefaces were introduced. It is thought to be derived from Iota, the smallest letter in the Greek alphabet, which was used as a diacritic below other vowels in ancient Greek texts; others say it is derived from yodh, the smallest letter in Hebrew and Aramic alphabets.
2. A very small part or quantity; a particle, jot, or whit.
Arthur: Hey, Alfred, did you know that the dot over an "i" or "j" is called a tittle?
Alfred: Too bad I care a tittle.