The oldest known dictionaries are cuneiform tablets from the Akkadian empire with biliingual wordlists in Sumerian and Akkadian discovered in Ebla in modern Syria.
The Urra=hubullu glossary, a major Babylonian glossary or encyclopedia from the second millenium BCE, preserved in the Louvre, is an outstanding example of this early form of wordlist.
"The canonical version extends to 24 tablets. The conventional title is the first gloss, ur5-ra and ḫubullu meaning "interest-bearing debt" in Sumerian and Akkadian, respectively. One bilingual version from Ugarit [RS2.(23)+] is Sumerian/Hurrian rather than Sumerian/Akkadian.
"Tablets 4 and 5 list naval and terrestrial vehicles, respectively. Tablets 13 to 15 contain a systematic enumeration of animal names, tablet 16 lists stones and tablet 17 plants. Tablet 22 lists star names.
"The bulk of the collection was compiled in the Old Babylonian period (early 2nd millennium BC), with pre-canonical forerunner documents extending into the later 3rd millennium" (Wikipedia article on Urra=hubullu, accessed 05-08-2009).