This poem is a reminder that oaths taken under passionate embrace are not always kept later:

Japanese	Romanization	Translation
ちぎりきな	Chigiriki na	But we promised!
かたみに袖を	Katami no sodé wo	While wringing out tears from
しぼりつつ	Shibori tsutsu	Each other’s sleeves,
末すえの松山	Sué no Matsuyama	That never would the waves wash over
波こさじとは	Nami kosaji to wa	Sue-no-Matsu Mountain.

As Mostow notes the author, Kiyohara no Motosuke (清原元輔, 908 – 990) 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. Motosuke is also one of the Thirty-six Immortals of Poetry.

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. We see another example of a poet writing on behalf of another in poem 59. Still it serves as a sobering reminder that passion might be wonderful at the time, but is fickle too.

P.S. Speaking of pines, poem 16 and poem 34.