This poem is a reminder that oaths taken under passionate embrace are not always kept later:
|ちぎりきな||Chigiriki na||But we promised!|
|かたみに袖を||Katami no sode wo||While wringing out tears from|
|しぼりつつ||Shibori tsutsu||Each other’s sleeves,|
|末の松山||Sue no Matsuyama||That never would the waves was over|
|波こさじとは||Nami kosaji to wa||Sue-no-Matsu Mountain.|
As Mostow notes the author, Kiyohara no Motosuke was the grandson of Kiyohara no Fukayabu of poem 36 and also the father of Sei Shōnagon who authored the Pillow Book and poem 62 in the anthology. Motosuke is also one of the Thirty-six Immortals of Poetry as well.
Sue-no-matsu is an actual mountain in Japan in Miyagi Prefecture, called sue no matsuyama (末の松山). This same mountain is said to have been visited by the Haiku poet, Basho, in a later age. The term matsuyama here (松山) refers to pine-clad mountains, so the idea is that the mountain will never wash under the waves, and thus the lovers’ feelings for each other would never die.
The poem’s intent here, as stated by the author himself in writing, was not to express Motosuke’s feelings, but rather for a friend whose lover’s feelings seemed to have grown cold. Still it serves as a sobering reminder that passion might be wonderful at the time, but is damnably fickle too.
P.S. Speaking of pines, poem 16.