The ann-thennath can be broken up as follows: Sindarin ann = long, thenn = short, -ath = collective plural. Thennath could be related to the Elvish root ten. In this case, the ann-thennath could mean long lines.
The English metric mode of Beren and Lúthien consists in a iambic tetrameter (four pairs of unstressed and stressed syllable); in Classical Greek poetry however, the iambic tetrameter was originally 4 pairs of alternating short and long syllables, which is consistent to the meaning of the Sindarin term ("long-short"). This difference of terminology might account for Strider's adaptation/rendition from Sindarin (quantitative verse) to Westron (accentual-syllabic verse).