Fimbrethil is Treebeard's long-lost wife. The pair were beloved since before even Morgoth first arose in power during the youth of the world. Translated, her name meant according to the 1966 Index 'slim-birch' (according to Appendix F 'slender-beech'). As with all of the other Entwives, Fimbrethil had been missing since Sauron's forces destroyed the gardens of the Entwives during the Second Age. At the time of the War of the Ring, Treebeard had not seen his beloved Fimbrethil for over 3,000 years.
Many other races wrote tragic songs about the loss of the Entwives and the remaining Ents' sad search for them; the Elves made many songs about it, one of which Treebeard recited to Merry and Pippin.
Treebeard said that the Ents made no great songs about the lost Entwives; they were content to chant their beautiful names under their breath whenever their thoughts turned back to them.
During the Last March of the Ents against Isengard, Treebeard felt that many of the Ents were marching to their doom, and that indeed, he might never see his Fimbrethil ever again. Treebeard told Merry and Pippin that Saruman had to be stopped, although he would dearly have liked to see Fimbrethil once again.